The W66 thermonuclear warhead was used on the Sprint anti-ballistic missile system, designed to be a short range interceptor to shoot down incoming ICBM warheads.
The W66 was a low yield (details not declassified, but reportedly a few kilotons) thermonuclear bomb. It was designed to destroy or disable an incoming warhead using neutron flux, and to a lesser extent X-ray and blast effects.
The W66 was 18 inches in maximum diameter and 35 inches long, with a weight of approximately 150 pounds.
References
W66 Wikipedia(Text) CC BY-SA