I'm inclined to feel neither too big nor too small would be good as far as class-size is concerned. Of course, "too much," either way, will be determined subjectively, but something like 200+ strikes me as too large to encourage much professor–student interaction (then, maybe some students wouldn't desire any); a number in the twenties, on the other hand, could easily come to be unbearable if one doesn't like enough of one's classmates (or if one really dislikes even just one or two), as one will be stuck with maybe the entire group for at least a couple of years (after which, I suppose, one would be moving out of the classroom).