I bought an S (size 5/6) for my 4yo who wears 5 in most brands. Length is perfect with the loop on.
The dress is very nice. It's far from "quality", but fair for price, and sure much better built than the average costume.
It comes with an adjustable headband and gloves (half arm covers). The gloves' elastic is a bit tight for us, but the headband works great for a princess updo (we chose to make a Belle hairstyle, and used the band double folded in a chignon).
I think that any little girl will like this dress. My daughter is not much of the princess type, but she was blown away when she tried this gown on. It's been hard to convince her to wait until party day.