Tried a rough simulation, creature's about half the size of his pupil, which doesn't look super dilated or anything so it's let's say... 4-6 mm in diameter -> creature should be like 2-3 mm tall
If you draw a 2-3 mm stick figure and hold it up close to your eye it looks pretty small (but also hella blurry.)
Assuming a little creature that was actually on your eye would be less blurry I think it would be about the size depicted in this chapter? Small in a way where "it's the size of a human and far away" would seem reasonable.