Fits pretty true to size. I'm 5' 8" and muscular, usually wear size 2-4 in dresses and I ordered a small. I still have enough room to wear a small sweatshirt or longsleeve underneath. The jacket has zippable vents at the underarms and a zipper on the front to attach a vest (although I'd probably size up if you're planning on wearing a thick/down vest). It really helps break the wind too.