6X is the size for girls who haven't outgrown the babyfat stage, with a little more room in the hip/rear end. So that size is between 6 and 7.
If you go to the Lands' End website, you can print out a shoe sizer. They also have lots of plus and slim sizes.
My Emma, who is 7, needs a 6 waist and a 7 length, so clothes shopping is pretty frustrating. We try lots of stuff on.
It would help if she would agree to wear overalls because then we wouldn't have to fit the waist, but she refuses.
I like Zappos for shoe purchases because they have free shipping AND free returns.