Maybe a better comprimise would be for her to pick the style and for you to pick the size.

Here's another idea - let her pick out the outfit and size, and then you buy the identical thing for her the next size up. Take both sets home - let her try on both, take pictures, and show her how the bigger size actually makes her look smaller and fitter. Then return the one you don't want.

Also, to help get rid of the really old stuff - make a new family rule that if someone gets something new, something old goes to charity.