I usually wear size 10-12 but based on several reviews, I ordered a size large 14-16.  It fits perfectly.  The medium would have been too tight.  For the price, it's really a nice costume.  I am only 5'1" so the dress is longer than I wanted, but I hemmed it on my sewing machine so now it's perfect.  The neckline on the white part of the dress was quite wide and didn't fit well, but I just took in two small tucks in the back of the neckline and then it was perfect.  The vest covered the tucks so you could not see them.  All in all, for the price it is a really nice costume.  I will wear it on Pirate Night on a Disney Cruise next week!