I recently ordered these when the girl and doll version were sold on clearance:
My daughter wears a size 8 slim, but I wanted her to be able to wear these for a bit. While I think the S would have fit, I'm not sure she'd have gotten through the winter in them. I got M and I'm glad I did; while she has grow room, she isn't drowning in them. There is a fold on the sleeve and she does a single foldover of the waist so they aren't too long.
The pajamas are adorable and she absolutely loves them. I'm not sure how they'll stand up to repeated washings; I've only done them once in the delicate cycle and hung them to dry and they're fine. I'd prefer cotton, but I understand that there's a fire-retardency law concerning children's PJs in the U.S. I'm glad that I paid $28 for them - I like them but not for full retail.