I am 5'10, 130 pounds, bra size 34C and I ordered a size M in the 'Smocked bikini top and bottoms.' I'm typically a size 4 or 26 waist in pants. I didn't want the bottoms to be too snug and reading the other reviews it seemed like this bikini ran a little small and it DID. I'm so glad I went with the M it fit perfectly. If you have a smaller chest and you don't mind the bottoms being a little snug, then I would order the S. Shipped to me in 6 days and I didn't even get the express shipping!