I am 5'9" and approximately 125 pounds. I bought a small and it seems to work perfectly. I think the shirt is generally pretty comfortable and I love the way it looks. The only concern is it's a little tight around the armpits, like the arm holes were cut a little small or something. Maybe that would be fixed if I sized up, but I am pretty skinny, so I don't feel that sizing up would be a good fit for me. Other than that, the tank is great and I have bought a couple different colors.See full review