If you or your daughter is a little shy about wearing a long dress that is sparse on the top make sure to look at ones that come up a little bit further on the back. Many floor length dresses are bare backed and the bottom of the back comes down to the waist, make sure you are comfortable with the amount of skin you are revealing and that it is in good taste.
The color black has a slimming effect and hides a multitude of sins. It makes the hips look narrower and hides any unwanted bumps. In some styles a black prom dress might even make you look taller if you are very short. You can put color in accessories and shoes and look very sophisticated and tasteful.
This is just a sampling of designers as there are many from which to choose. Whatever you decide to do make sure you take your time look at a few options to make sure it is the very best for your tastes and budget. There are so many dresses to choose from sometime it can be daunting.