We ordered a medium super girl costume for our 6 year old.  She normally wears size 6-7 shirts from the Gap but she is also tall.  The boots are very flimsy and don't stay up but the rest of the costume is great and fits well (the skirt is short but she only wears it indoor, she will need tights if she wants to wear it out so areas are covered properly).  My daughter loves to dress up and has had lots of fun using this costume.  So far (after 3 weeks) the costume still looks great, there is no damage to it and she sometimes wears it all day long.  Overall very cute!