As NG said, go to Victoria's Secret to get measured. That is what I did after I lost my weight and I have been happy with the size they said I was...well after the initial sadness of dropping from a C to an A cup.
Anyway, the other option is try on a variety of bras around your size and see what fits and feels best. Good Luck!
Edit: Ok, so I measured myself and my bra. I measured under my breasts at 27 in. My bra measured 27 in. I was 33 in the cup area which is 1 inch difference from 32 which is 27+5. My bra size is 34A. 27 + 5 doesn't equal 34. 2 inches off. Guess I could try a 32A the next time I bra shop, but it probably wouldn't fit since the 34 is 27 inches which is what I need. Who the hell knows?
