The plate scale p of a telescope connects the angular separation θ of an object with the linear separation s of its image at the focal plane
If F is measured in mm, the plate scale in radians per mm is given by p=θ / s = 1 / F, since s=Fθ.
Plate scale is usually expressed in arc-seconds per mm p=206265 / F, where F is in mm, or expressed in arc-seconds per pixel after further division through the pixel scale.
Plate scale on JWST FGS/NIRISS
The plate scale of the James Webb Space Telescope component Fine Guidance Sensor and Near Infrared Imager and Slitless Spectrograph is about 0.065 arcsec/pixel. It uses a 2048 x 2048 pixel array with a pixel size of 18 microns a side with a field of view of 2.2' x 2.2'
References
Plate scale Wikipedia(Text) CC BY-SA