I'm usually a Small in dresses and shirts, 3 in jeans, 5'5" and about 140lbs (mostly muscle lol). I ordered a Medium and I'm glad I did! It first really well! I have long legs so usually rompers fit too short in the crotch and butt area and this one rides a little bit but not too uncomfortably. The arms and from the waist down is kind of like a thick sheer material so I'd wear some spanks under probably. The top half of the romper is double layered with a non see through material so that's fine. I love it!!