I am a Med (5'3", 140lbs) but the fit was a little tight. I'm 34B and even though I could button it, it stretched uncomfortably across the chest. If you want to wear it open or halfway buttoned, then your true size is fine. If not, then go up a size. I love this vest and bought another vest that is similar. I am returning this and bought a Large before it sold out.

Too cute