I think 8inches is really big and really hot. 8 and a half is jumbo, 9 is scary but ok, 9 and a half is borderline freakish, and 10 is "stay away"
Depends on the context too. If I'm walking around on a nude beach, I'd want a fully limp cock measung 7" or so slapping around between my legs. That would be an excellent size.
A tiny little 5 foot tall girl would probably say that's way too big, and some 6 foot 4 inch tall african goomba girl amazon woman might think it's just right.
Along the same lines, a dingle dangle that is 5 inches HARD would be very unimpressive at a nude beach, the short girl would like it, and the big woman would laugh.