Sizing:
Bras are notoriously hard to fit. Cup size, frame size, and bust measurements all come into play. I have by-passed most of the confusion by making the back and top ties adjustable and by making the space at the center front adjustable as well. Most dancers will get a good fit if you order your regular cup size, or order one cup size up and pad it as described below. If you have a small frame and a large bust,  ie 30"-34" C-cup, 30-36" D cup, or 32-38"  DD cup, you may want to order one cup size smaller for a better fit.
If you will be wearing a choli under your coin bra fit is not as much of an issue; as your choli will be holding you up and in. Be aware that your own taste comes into fitting a costume bra as well; one dancers pleasingly abundant decolletage is anothers - oh my goodness Im falling out of this thing! If you really really can't decide between 2 sizes there is a long distance try on option: you can order bra forms in both sizes you are considering. Send them back  for a credit towards the coin bra in the size you decide on!

Here is how to measure for cup size:
1) Start by wearing a supportive bra. Measure around your body by placing a tape measure on your back above your bra line Then bring it to the front above the fullest part of your bust.
2) Measure the fullest part of the bust. The tape should go from the back bra line to the front. It should be parallel to the ground and measured loosely.
3) Simply deduct the smaller measurement from the larger measurement. This is your cup size. 1"= A cup, 2" = B cup, 3" = C cup, 4" = D cup, 5"= DD cup