Plate scale (or image scale) is a measure of telescopes that produce images on focal planes, such as for photographing (e.g., on photographic plates). It is the relation between angle across the celestial sphere and distance across the image. Effectively, it is a measure of magnification or what's termed power for binoculars and backyard telescopes, but oriented toward imaging, etc.
A plate scale does not fully determine the telescope's angular resolution, which more basically depends on the consequences of refraction, i.e., Airy disks.
The plate scale is proportional to the reciprocal of the telescope's focal length. Equations:
p = θ/s ∝ 1/f parcsec/mm = 206265/fmm