I ordered this for an away family member for an upcoming event and then to carry over into this year's Halloween festivities. She wears a size 7/8 so I ordered a medium. It's a perfect fit. Wish they had given a tiny bit of room since it is a costume. Others say they ordered to fit and it was perfect but the skirt raised when the child bent over.
I decided to order a 16" tulle to add a little length and stretch the costume life, fingers crossed that it works.