I usually buy a small in bras because I have small boobs. I ordered a medium and small too see which one fit better because other reviewers mentioned how tight it is on. The medium was tight but that worked out for me because my boobs are also two different sizes and it is quite noticeable. Plus I imagine after some wear this will stretch out a bit.

I'm 5'7", 140lbs and wear about a size B 34/36" in bras.