I ordered Child Small (4-6) for my almost 6.5 year old. She is on the trim side, so the costume fit her loose. As far as length is was barely long enough to touch her shoes. I am glad I ordered this size, even if she will wear it only for couple more weeks, because I am sure the next size would be too big for her and she would feel uncomfortable in it and the effect would not be the same. She needed it for a performance.
It is made out of polyester so she felt warm in it pretty quickly. Still everybody loved her kitty outfit!