In Cleveland, Ohio, high school senior Wren DeSantis opening narration describes Halloween in Cleveland as a time for its citizens to dress up and be less ashamed of themselves, citing the nickname "the mistake by the lake." This Halloween though would be the first that she and her family spend since the death of her father, a former Def Jam Records production assistant. She also focuses on the fact that she, her mother, Joy, and 8-year-old little brother, Albert are touched by the death of her father in their own way. Albert, who also has reputation as a legend at video games, has apparently chose a life of elective mutism. Wren's widowed mother, Joy, dyed her hair blonde and is dating a 26-year-old boyfriend named "Keevin." Wren has her heart set on college life at New York University. Her friend April has her eyes set on social status, and nerdy friend Roosevelt has his heart set on Wren. The two girls are surprised when they find they are invited to a Halloween party by local heartthrob Aaron Riley, even as she performs a seemingly uncool "explaining rap," about her friend Roosevelt's proposed E.O. Wilson costume. Wren decides on a Dorothy Gale costume, and April decides to dress as a sexy cat. Albert wants a costume with a fake severed arm, while Wren tries to talk him into selecting a Spider-Man costume. He chooses both. On the day she is supposed to go to the party, she is also ordered by her mother to take Albert trick-or-treating so she can go to a party with her boyfriend. Despite Wren's pleas to not watch her brother, Joy offers to sign her applications for NYU if she does. But, Wren loses Albert, and runs through a haunted house, desperate and very scared. The party that Keevin invites Joy to turns out to be run by Keevin's friend Nate Brueder who is roughly the same age. None of the people at the party even recognize that she is dressed like Britney Spears during the early period of her career. Brueder and friends engage in behavior such as binge drinking, farting in her cell phone, and causing other juvenile chaos.
At a haunted house later that night, Wren and April run into her nerdy friend Roosevelt and his friend Peng, dressed as Aaron Burr, complete with an 1814-style pistol. During this time, she finds out that Albert has wandered, so she must find him. On his own, Albert meets a convenience store employee named Fuzzy who seeks revenge against Jörgen, a mixed-martial arts fighter, who won the heart of his ex-girlfriend Lara. Recognizing Albert for his well-known reputation from a video arcade, he seeks his help to get even with his ex, and the boy accepts his offer and friendship. Wren and April just miss him at the convenience as Fuzzy drives off with Albert, and they both encounter Roosevelt and Peng again. Wren coerces Roosevelt into driving his mother's car around the city to look for her little brother. Roosevelt's parents are semi-eccentric pacifistic lesbians, and Wren previously thought his reference to them as his "Moms" was an attempt to mimic rappers such as Li'l Wayne. Roosevelt asks his mothers to borrow the car, but when they insist that he asks it in ancient Greek; he does so, and they say no. Citing his lack of ability to translate the word "seat belt" into ancient Greek, he sneaks the keys to the car and drives off with them anyhow. The car is a yellow 1986 Volvo 245 station wagon with mechanical issues. Peng turns on the car stereo, which is set to an NPR station and starts messing with it, and as he and Roosevelt fight over it, the knob breaks causing it to blare Josh Groban's cover of "You Raise Me Up" at full volume, thus making them appear uncool as they roll onto a street that's popular for kids who cruise with their cars at night. And to make matters worse, the car stalls right in front of the SUV driven by Mike Puglio the "Wedgie King" and his sidekick dressed like The Incredible Hulk. Both harass the four kids, until one of them bangs on the hood, causing it to start up again, giving them an opportunity to escape.
Across town, Fuzzy and Albert arrive at Lara's apartment building, and he climbs the building to throw toilet paper all over her apartment while Albert keeps lookout for the police, only to find that he not only threw it into the wrong apartment (Lara's was the one next to it), but he also accidentally set it on fire from some candles. The owners of the wrong apartment turn out to be a Samoan couple who begin to beat Fuzzy, but he manages to escape. During this time a police tow truck confiscates his car for parking in a tow-away zone, not noticing what's going on in the building, but not before Albert climbs in the back seat to retrieve his Halloween candy. Fuzzy tells Lara about Albert and how his car was towed away. She doesn't believe him, but goes along with it, anyhow. Wren suspects that Albert might be at a local Captain Chicken restaurant, a pirate-themed fast food chain that has a giant statue of a chicken with an eye-patch and a hook on one wing. Again they arrive just too late to find him, almost running him over while fleeing Mike Puglio before he is rescued by Denise, a young college girl dressed in a "Galaxy Scout" (a fictional anime character) costume and is invited to another party. The four kids ask the girl in the drive-thru window about Albert's whereabouts, but have no success. While leaving, they find their way out of the drive-thru blocked off by Puglio. Roosevelt tries to get away from the bullies, but instead he crashes the car into the statue of the chicken, which lands on the car and appears to be engaging in doggy-style sex with it. The whole incident embarrasses April to the point of tears, provoking Peng to get out of the car and challenge the bully to a duel. Initially, Puglio does not take the challenge seriously, believing the gun is not real until he fires it at him, blowing apart a chicken drumstick Puglio used as a mock weapon. The drive-thru girl calls the police, and Puglio and his friend flee the scene while April escapes. Police officers arrive and try to arrest the three of them, but they drive off only to find their patrol car covered with toilet paper.
Albert seems to be having a good time with Denise and her friends, until Jörgen crashes the party and takes some of his candy, throwing most of it on the ground, and tries to dance with the girls at the party, who throw him out. Remembering Fuzzy's description of Jörgen, Albert stows away in his convertible, just as Fuzzy and Lara arrive at the same party, which Lara wanted to attend. Denise and her friends tell them that they were partying with Albert, and Lara soon realizes Fuzzy was telling the truth, just before Jörgen throws Albert's Halloween pumpkin basket at Fuzzy, who suddenly spots Albert in the backseat of Jörgen's car, evidently with his own revenge plot. Wren, Roosevelt, and Peng drive off looking for both Albert and now April, who later calls from Aaron Riley's party with the false impression that she has found Albert. Wren, Roosevelt, and Peng arrive at the party, and for at least this night, the earlier duel gives Peng some level of social acceptance, whereas he had previously became convinced he and Roosevelt were pre-destined to be social outcasts. Wren calls the police to ask if Albert has been arrested, since he gets into a lot of mischief, but Albert was still out there. Later, April fulfills a promise she made earlier to Peng to let him touch her breasts. When he gets nervous about the chance to feel her up, she grabs his hand and puts it on one of them herself, but still tries to conceal her own desire for him by slapping his face in case anybody saw them together.
Seeking to escape Keevin and the party, Joy tries to find the bathroom and unexpectedly stumbles upon Brueder's parents. All three share a cup of tea, and Joy tearfully confesses that she's dating their son's friend not only to compensate for the loss of her husband, but also the potential loss of her daughter to college and her son to silence. Eventually she leaves the party and returns home, discovering the children are not home. Jörgen arrives at his home, unaware that Albert stowed away in his car. Soon after, Fuzzy arrives, and he and Albert hatch a plan to get revenge on Jörgen by leaving a flaming bag of dog feces and fireworks on his front porch. When Jörgen investigates what's going on, the bag explodes, hurting his foot. He spots Albert but not Fuzzy. Albert attempts to run away, but Jörgen catches him and locks him in his bedroom, leaving Fuzzy to come up with a rescue/revenge plan. Back at Aaron Riley's party, just as Roosevelt is about to confess his love for Wren, Aaron Riley sings a song dedicated to Wren, but she turns him down and tries to chase Roosevelt as he drives away in frustration. Just then Wren receives a phone call on her cell phone from Jörgen, who is holding Albert hostage at his house, and threatens to turn him over to the police for pulling the stunt with Fuzzy, unless she gives him $400 in cash, which she doesn't have. After Jörgen gives her his address, Wren desperately runs through the streets of Cleveland trying to find his house. Arriving there and telling Jörgen she doesn't have the money, he calls the police but puts them on hold when Wren finds out that he is a big fan of the Beastie Boys, and offers to give him the collectible jacket she inherited from her late father which was left behind by Mike D as a substitute for cash. Jörgen seems satisfied and takes it as a better offer, but he decides to turn them over to the police anyhow. Just then, Fuzzy breaks in to rescue the boy and his older sister, by throwing bricks and fireworks throughout the house. Jörgen is slightly injured in the explosion. Before the police arrive, Fuzzy strips down naked and runs off in order to divert attention from the two kids.
Wren and Albert visit the cemetery where their father is buried and place a plastic pumpkin with flowers on his grave. Albert thanks Wren for finding and rescuing him, just before their mother picks them up and drives them home. It is morning when they arrive home. Upon their arrival, they find Roosevelt trying to prevent Wren's mother from entering her house without the opportunity to profess his love for her, something which he was encouraged to do by his parents despite coming home with the Volvo severely damaged and confessing to them about the night's events. Meanwhile, April wakes up to find herself in the arms of Peng, and after looking around for witnesses decides to make out with him again. Fuzzy meets Denise, who asks him out on a date. Fuzzy also reveals that his real name is Manuel. Later, Albert reveals that he secretly has been pulling prank phone calls on both his mother, sister, and his sister's friend for six months, which includes the uploading of a video to Wren's Facebook page, making fun of Wren's bug dance and "explaining rap" that she did during the period of Riley's invitation.Victoria Justice as Wren DeSantis, Albert's sister
Jane Levy as April Martin, Wren's best friend; A persistent social climber, she would rather drag Wren to a cool kids' party over finding her brother.
Thomas Mann as Roosevelt Leroux, one of the nerds that helps Wren. He has strong feelings for Wren. Wren's crush.
Thomas McDonell as Aaron Riley, the most popular guy in school.
Jackson Nicoll as Albert "Spidey" DeSantis, Wren's oddball 8-year-old brother.
Chelsea Handler as Joy DeSantis, Wren and Albert's mom.
Osric Chau as Peng Chong, Roosevelt's best friend and fellow nerd who helps Wren; he has a crush on April.
Riki Lindhome as Denise, a young college girl in a Galaxy Scout costume.
Josh Pence as Keevin (pronounced Keeven, like "Steven"), Joy's 26-year-old boyfriend.
Johnny Knoxville (uncredited) as Jörgen, a mixed-martial arts fighter who is a bully to Fuzzy and Albert.
Willam Belli as Qwerty
Abby Elliott as Lara, Jörgen's girlfriend
Thomas Middleditch as Manuel "Fuzzy," a clerk whom Albert befriends, and Lara's former boyfriend
Ana Gasteyer as Jackie Leroux, Roosevelt's mom
Kerri Kenney-Silver as Barb Leroux, Roosevelt's other mom
Patrick de Ledebur as Mike Puglio
Holmes Osborne as Mr. Brueder
Annie Fitzpatrick as Mrs. Brueder
James Pumphrey as Nate Brueder, a friend of Keevin's who hosts a raging party in his parents' basement.
Peter Navy Tuiasosopo as Mr. Mahani (Samoan man)
Cooper Ross as zombie doctor
In January 2011, it was announced that Josh Schwartz would direct the film as his feature film directorial debut. By the spring of 2011, the lead role had been offered to Victoria Justice, and Jane Levy had entered talks for her role in the film, with Paramount announcing initial plans for the project to be shot in Minnesota. The location was changed to Michigan and later to Cleveland, Ohio. When the production moved to Cleveland, a home was scouted and selected in Cleveland, Ohio. In June 2011, Chelsea Handler entered negotiations to co-star in the film. The film was released on October 26, 2012. Director Josh Schwartz discounted Internet claims that he was playing a convenience-store clerk. "No," he said shortly before the movie opened. "I get asked about it a lot, but that's an IMDb mistake. There are convenience-store clerks in the movie – just none played by me!"
A music video of Carly Rae Jepsen's "This Kiss" was shown prior to the film's release in theaters. Fun Size opened at #10, earning $4.1 million over its first weekend at 3,014 theaters and averaging about $1,361 per venue. The film earned $10,911,519 and is the lowest grossing wide released film from Nickelodeon Movies. Fun Size was released on DVD and Blu-ray on February 19, 2013.
Fun Size received negative reviews from critics, several of whom criticized the adult humor and sexual content despite the film's Nickelodeon pedigree. Rotten Tomatoes reports that of 25% of 72 surveyed critics gave the film a positive review; the average rating is 4/10. The consensus states: "It occasionally shows surprising flashes of wit, but Fun Size is too safe and formulaic – not to mention unfunny – to survive comparisons to the '80s teen movies it eagerly imitates." Adam Graham of Detroit News gave this film a score of C and said that "it feels like a project that was pitched for the CW but watered down for Nickelodeon audiences. In the end, everyone loses." Jen Chaney of Washington Post gave this film a score of 1/4, calling it "a 90-minute theatrical release from Nickelodeon Productions that, if anything, should have aired as a half-hour Nickelodeon special." David Martindale of Dallas Morning News gave the film a C+, saying, "like a 'fun size' chocolate bar, it's just empty calories. It's a momentary pleasure, instantly forgotten." On the positive side, Sheri Linden of The Hollywood Reporter enjoyed this film and wrote, "Though it doesn't always hit the hilarity target, this tween-targeted romp strikes a sweet-but-not-sappy balance."