Hi, Lisa!
It is definitely a lot of fun to do some serious shopping while on vacation!
I'm going to be really honest with you. I have not seen anything larger than a 2x available at any of the stores at the Walt Disney World Resort. They always have the cutest shirts and I would wear them in a heartbeat, but even the 2x they carry runs on the small side.
The only retailer I can confirm that carries sizes larger than 2x is the Design-A-Tee store in Downtown Disney. I know you mentioned that you were looking for something nicer than a tee-shirt, but that's going to be tough. I truly wish I had better news for you. I tend to focus on all of the accessories like the fun hair clips, jewelry and purses while I'm shopping!
I have seen some very cute
women's size tops at the Disney Store that you might like. Something else you can do is purchase pretty shirts and add a little iron-on Mickey Mouse head decal to the collar or another fun spot on the shirt just to personalize it a little bit.
I'm so glad you stopped by the Moms Panel with your question, Lisa. I really wish there were more options. Please feel free to stop back by if we can help with any other questions you have!
Wishing you and your family a Pixie Dusted vacation!
~Jackie