57" is rather petite for an adult female. Chances are pretty good that an adult small will be too big on her. The measurements that the DisneyShopping.com website gives for a Misses small are:
- Misses Size: 6/8
- Bust: 34-35 in
- Waist: 26-27 in
- Hips: 36 1/2 - 37 1/2 in
No listing for height.
Maybe you could order the costume and wig but buy white leggings in her size. Use fabric paint to make the stitches on the legs. If the dress is too long, just cut off the excess at the hem. It's not supposed to look "finished" any way, so you wouldn't need to hem it.