Tops generally fit the size shirt you regularly wear. Waistbands are adjustable, so you want to fit the cups:

XSmall: 32-34A
Small: 32-34B, 32C
Medium: 36B, 34C, 32-34D, 32DD
Large: >36C, >36D, >34DD/DDD

If you have more questions about sizing feel free to send me an email at spirit.and.thread.crochet@gmail.com.