I would like to know the diameters of the smallest craters that can be clearly distinguished inside the black circle (see the quite large size image below).
I can estimate, but it is no more than that. From this page:
It is estimated that the diameter of the largest impact crater in the images is around 620 meters.
Based on looking at the uncropped images, that is the crater in your crop I think. So then doing some measurement from the image I make the smallest craters around 7m.
However this is confusing: the page I linked to above quotes a resolution of 0.7m which makes this very plausible. But the page you cited quotes the resolution as 7m, which would make these craters much larger than that (and therefore the big crater much larger than 620m).
I suspect but don't know that the '7m' resolution figure is wrong: it really is 0.7m. The reason I suspect this originates from this answer by uhoh. In that answer the resolution is quoted as, at an altitude of 265km 'better than 2.5m, in key areas better than 0.5m'. Well this image is from about 350km, so that gives a worst-case resolution of about 3.3m, and a best-case one of about 0.66m. Neither of these is anywhere near 7m, and the second one is quite close to 0.7m. So I'd guess (but it is no more than this) that the resolution is about 0.7m and thus a 7m diameter for these features is plausible.