We don’t work with common sizing guides, we like to have your exact measurements, even if you would like to buy a second-hand dress.
However, I’d like to explain how I sorted the second-hand and new dresses into the three categories Small, Medium and Large.

Small: The owner is either very young (junior I-II, youth dancer), or her breast size is small, around 80 cm, 31 1/2″, or her hip is narrow, around 86-88 cm, 34-34 1/2″.

Medium: The breast size of the owner varies, but the hip size is between 90 cm and 95 cm / 35 1/2 and 37 1/2″.
Some of the dresses are listed in both the medium and large categories.

Large: The hip size of the owner is larger than 95 cm / 37 1/2″. Again, you can find a few dresses here which are also in the medium size list, so you don’t miss a dress which could fit you.