I bought this shirt in grey, should have bought the red (i like bright colors more) but I wanted something neutral at the time so it's ok.
I am 5'6" 110 lbs with 34A and I bought the grey in xS. At first, the shirt was VERY LOOSE. I had to wash it 3 times with warm/hot water and machine dried it cotton (high) in order to shrink it a little. Fortunately, it did shrink but not as much as I'd like. It's cute and comfortable so I'm ok with the shirt. I use it as a shirt I wear to sleep or a shirt I wear under my sweater when days are cold.
Boat-necked style, sort of sheer, with ribbed fabric slabs on each side of my torso (from armpit to bottom length of shirt).
If you're a 34B or 34C and you weigh around 110-125ish lbs, you can buy the XS and fit it fine. 125lbs if you like to wear your shirts fitting and snug.