I'm 5'9" and 140 lbs, with a long-ish torso. I ordered a medium - annnnnd - I'm not gonna lie, my partner told me I looked like Jackie O. so to be cheeky and funny and fun I actually used this suit like lingerie the other night. ;) It feels wonderful on, and if your chest size is smaller (I'm 34 b, or so) you can cross the straps in back for a little more support. I agree with the last commenter about the shelf bra thoughts.
I have been looking for the perfect one piece to take me through Key West in a couple of months. I wanted something classic that wouldn’t give me crazy tan lines and could withstand activities like jet-skiing. This definitely fits the bill! The straps can be switched to cross in the back, which I didn’t realize until I received the suit. I was nervous about sizing, but this is honestly the most perfect-fitting suit I’ve ever purchased. The only thing I would mention is that the shelf-bra is not sports bra-level heavy duty and the pads within it are kind of small, I might slide some bigger ones in.