http://www.yuribou.net/blog/?p=276
I use this for size references.
While my personal favorite size is "K", everything on this site is acceptable in my terms. However, anything beyond "O" is more of a "meh" subject for me. As long as the breasts are perky, and not appear sagging, I'm alright with it.
Really, as long as they aren't as big as her body, it's all good ;D