Runs big!
I ordered a Medium at first (5'3 and about 135 lbs) which is my normal size, and I was swimming in it. I exchanged it for a XS and it fit much better with plenty of room. I am short-waisted, so the straps do slip off my shoulders sometimes. I may need to get it altered a bit, since I have them adjusted on the shortest "setting" with the buttons on the straps. But it's a cute, comfy piece for summer.