What about going to a department store like Carsons, JC Penney or Nordstroms? Talk to the personal shoppers at Nordstroms, your daughter could get a 3rd party opinion about what is appropriate and what will fit her.
I would also sit down with her for a heart-to-heart talk about how it doesn't matter what store your clothes come from as long as you like them, like they way they look, and feel comfortable in them.
Good luck!
My other suggestion is to go to these other stores without her, pick out a dress or two in her real size and have her try them on at home. That way, she's not "seen" in the store, she can try them on in the privacy of her bedroom and you can return the ones she doesn't like.