I'm 5'4" and 130 and fit. I bought a medium.  I am usually a size 2 or 4 in Banana Republic or Black House White Market. I love the length (at the knee or a tad below) and it's not to tight through the midsection.  Kind of a "sexy librarian" vibe. I was afraid to buy a small because I've had experience with less expensive clothes and they seem to be a bit tight through the rear and midsection.  Some don't mind but I would rather order up then alter it if needed.  I didn't need to with this dress.