I chose Medium (M) for my size 6 baby so I have extra fabric to turn into a Merida hunting dress. I fully expected to have to hem it, but while I had enough work to hem it above her ankles, the rest suited her slim figure. So I would suggest trying it on, especially if you don't want a tight fit. It worked great for the price and she loves it. Although the arms were two very different sizes (length and width): I may not have noticed it only on her, but it was very noticeable where I took the dress apart and put it back together.
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