I bought size 8 for my 8 year old daughter.  She's 48" and 52 lbs. They are a perfect fit - with plenty of room for growth.  We pull the straps all the way and they are boy cut (wide) on the sides.  Right now though - she wears a sweatshirt under and tucks it all underneath.  I can see her wearing these for at least two years and cutting them into shorts and wearing for another year.