Hi there! I am Pooh-sized, and we were in WDW in September. To echo what everyone else here said, WDW is extremely friendly towards "fluffy" folks, and you should have no problems.
If you would like to check out my trip report below, at the end of each post, I write a bit on anything that might be important for pooh-sized people to know from that day's adventures.
I found my biggest problem, by and large, to be the darn turnstiles. I sure wish they would do away with those things!! Talk about a recipe for embarassment.

I didn't have a problem getting into or fitting into any of the rides, just a bit of an issue getting OUT sometimes, especially on Space Mountain! Seats in some of the theaters can be a bit tight, you have to wodge yourself carefully into them, and if you sit down too quickly you can end up with nasty bruises on your hips from smashing them into the armrests. Interestingly enough I had the same problem with the desk chair provided in our room at Port Orleans!
Other then that, no issues. I bought
Crocs about 3 months before our trip and lived in them all summer long... my feet were very used to them and they were VERY comfy. I used to suffer with a lot of foot pain in the mornings and after walking a lot (
plantar fascitis) but since I started wearing Crocs all the time i have NO problems with that at all. If you are new to Crocs, I would suggest getting them well in advance of your trip and getting used to them BEFORE you do all that walking. My DH decided to buy a pair of the POTC Crocs IN WDW and tried to wear them for 14 hours the next day and they gave him blisters. DUUUHH!!!

Now that his feet are used to them he wears them all the time and loves them.
I did have issues with the seatbelts on the airplanes, but I asked a flight attendant for an extender, and, she was very kind and prompt in getting me one. NO WAY would i have been able to get that puppy latched otherwise. That was on Delta, FWIW.
Enjoy your trip, and don't worry! I stopped being self-concious about myself at all after seeing a VERY AMPLE young lady at Typhoon Lagoon who removed her cover-up and was wearing a teeny little string bikini with a THONG behind. YIKES!!

She needed to have gotten a second opinion on her bathing suit choice...