I'm 5'3, 105lbs and bought a size small. (I'm a size 4 American Eagle jeans / size 0 BKE for reference) The Pros: I carry any body fat I have in my thighs/rear and this provided ample coverage. Its hard to find that in a swim bottom these days. They felt secure, were made well, and the fabric was good.  The Cons: I have a super flat stomach and felt like the band wasn't very flattering. I felt a bit like a squished sausage casing.  If I'd had the opportunity to try on in a store I might have bought a medium instead, but I didn't love them enough to try to order another pair.