If you were looking for, say, a 1m per pixel quality, what focal length/sensor size would you need? Would there be any off the shelf/commercial solutions?
The best angular resolution achievable for a given telescope diameter is determined by the diffraction limit. For visible light at 500 km distance this works out to a minimum aperture diameter of about 40 cm to achieve 1 meter ground resolution. That's assuming a perfect optical system, so you might need a somewhat larger lens or mirror in reality.
Atmospheric seeing conditions, solar illumination angle, attitude stability are a few more of the many considerations that go into the overall system design.