Hi Jennifer!
The dresses that are offered at Bibbidi Bobbidi Boutique go up to a size 12. Here are a few ideas for you:
You are not obligated to buy the dress there. If you think that these sizes may not work for your daughter, you are absolutely welcome to bring a dress that your daughter will be comfortable in. It does not necessarily have to be a princess costume for her to look like a princess.
Another idea is to try ordering an adult size small costume. Check
www.disneystore.com, or call Walt Disney World's Merchandise and Gift Services at 1-877-560-6477. If it's available, they'll find it. Their offices are open on Monday through Friday, 9am to 5:30pm.
The phone number for Bibbidi Bobbidi Boutique is (407)WDW-STYLE. The Fairy Godmothers-in-training are fantastic and will help you in any way they can.
Good luck in your search! Your little princess will love this enchanting experience!