In real life, honestly I prefer flat or at most petite. I mean, I don't mind bigger, but I find it much easier to talk with them that way. Otherwise I'm fighting the urge to glance/stare the whole time. :l
I'm somewhere between "large" and "big" on your scale.
The thing to remember, though, is that boobs look bigger in three-dimensional Real Life than on a flat page or screen. It takes more exaggeration when drawn to make them seem big. If you had some statue or something in real life to show me, I'd probably go down a step or two on your scale.