Anyone out there who is worried about not fitting on the rides at WDW the following is a report of what I experienced on my trip about 2 weeks ago. First of all I am extra pooh sized (lol) and usually wear a womens size 26/28 however sometimes buy a 30/32 depending on how the item is made.
Magic Kingdom- I rode EVERY ride at the park except for Dumbo, Astro Orbitor, and the carrosel. The only reason I didn't ride those was because we just had no interest in them. The only "problem" I had was on space mtn and I really wouldn't even call it a problem. The ride was easy for me to get into (regardless of which seat because I rode it several times and tried all 3) however it is difficult to climb out of without looking a little silly. If you have trouble getting up when you sit on the floor you may have trouble getting out of the ride because you sit so low to the ground.
MGM- I rode EVERY ride at this park with no problem at all
Animal Kingdom- I rode EVERY ride at this park except Kali River Rapids and that was because I've heard too many bad reports about the seat belts on this ride.
EPCOT- I rode EVERY ride at this park except Mission Space and thats because I was afraid of getting sick.
SOOOO if you are my size or smaller have no fear because you will be fine. This was my 4th trip and the first time I had ever ridden Space Mtn., Test Track, Rockn Rollercoaster. or Expedition Everest because I was always terrified I wouldn't fit. This trip I just said "I don't care anymore I'm going to at least try". Now I regret all those past trips I spent worrying about not fitting and I would have been just fine all along.
A tip that I did follow that may or may not have helped was to always pull the seat belt all the way out before sitting down and then clicking it. On test track I did ride in the front middle seat because it only had a lap belt and not a shoulder belt. I don't know if it mattered or not but I had read somewhere to try that so I did and it was fine.
As far as the rides with the lap bar such as: Expedition Everest, Space MTN, Big Thunder, ect. it seemed to me that the bar just stops whenever it reaches your waist. I don't think anyone could not ride those rides unless their butt just didn't fit in the seat. Believe me I have a big butt and mine fit just fine. Also the shoulder harness on Rockn Rollercoaster is nothing to worry about either. It just stops when it meets your chest, nothing like you see at these other amusement parks. Their is no belt that you have to click in place like at Universal just the shoulder harness.
If you have any questions at all please feel free to pm me and I'll try to help. I would hate for anyone to waste their trip like I used to do just worrying about something like this. Go and have a great time. You will be fine!
Oh and I also suggest the crocs. I wore my croc athens flip flops every day and they were GREAT!