There is no "ideal" because its all about proportion. A D cup will look massive on a tiny girl, and look like nothing on a bigger girl. So it really depends on the size of the girl in question.
Squally likes them small and perky. I can't argue with that since one of my buddies is just wild about perky A's. Never bigger than a B for him. Me? Bigger the better and fake is fine. Just put them on a slimmer waist for that hour glass figure. Merilyn Sakova bouncing and swinging them is my joy. Wifey and Kelly Madison make me jump too.