I know boys thing about this a lot but actually it is really hard to say what a girl's breast size is like AA, A, B and so on. Depending on the design of the bra you wear different sizes work. Your boobs change size during the month. Also I think depending on the shape of your boobs different sizes might look better or not so good.
So I think that if men were to think about this more they would think about what sort of boobs they like is about more than just size but lots of other things as well