The largest size costume disney sells is a 14 but the age limit for wearing costumes in the park is 10. You may want to look at getting a cute fun outfit and doing the crown or coach package at BBB.
The costumes I have posted on the front page of that thread are currently being changed for anyone interesed in costumes. So far the Ariel dress has changed and the others are making their way out.
Elmo888 is correct disney doesn't want the liability of what may happen if someone confuses a guest for a character.
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