For reference: I am 5'4" (without heels on), around 115lbs, 32/34C bra size, not very curvy, and I ordered the size Small. I think the Medium would've fit me as well, but I wanted the fitted bandage look. The great thing about these dresses is that any body shape can wear them. If you're a straighter build like me, it will give you curves in the right places. If you are a curvier build, it will hug your curves and smooth out any areas that you wish to smooth.
Good luck finding your perfect dress!