Digging up an old thread to add a few new thoughts.
I just got back from 3 days at the parks. I'm pretty big, it's a tight fit on a coach airplane seat and I need a seatbelt extender. Size 28 woman, with a lot of it on my bottom half.
Here's a brief, whirlwind synopsis of my trip:
MGM Studios was fine for turnstyles. Sounds Dangerous seats were small, but you can slouch on the front edge comfortably (it makes sense if you're there). Muppets was a bit tight. Seatbelt in Tower of Terror was huge, and I sat on the aisle. Star Tours seats were okay, didn't use the belt (I went on just the movie, not the motion simulator aspect, I get motion sick). Other rides I went on were all fine with the seats: One Man's Dream, Millionaire, Backlot Tour, Beauty and the Beast, Great Movie Ride, Indiana Jones, Lights Motors Action, and Mermaid.
Magic Kingdom was fine for what I went on but the turnstyles were a squeeze in most places. I navigated them okay (if not a bit gracelessly) except for coming out of Small World. There, I took the adjacent handicap exit, as I could tell just starting to edge in that it wasn't going to work. Seats for PhilHarmagic were fine, they were pretty comfy, like a normal theatre seat. Other rides I went on were Stitch, Buzz Lightyear, Transit Authority, Peter Pan, Snow White, Pooh, Small World, Haunted Mansion, Jungle Cruise, Pirates, and Big Thunder Mountain Railroad. Except for the turnstyles being snug, the rides were fine.
EPCOT I only went on a couple of things. France movie seats were fine, Malestrom was fine, Spaceship Earth was fine, and Soarin' was fine! Since I couldn't find Soarin' information on WDW At Large, I was worried about it. I'll quote what I submitted to the Allsears site about it below (possibly slightly misremembered, based on what I see in the picture, I guess there wasn't a cushion but a more 'seat proper' part of the seat):
Soarin' was one I was scared to go on, just because I was unfamiliar with it from the reports. I decided to give it a go, and it was NO problem whatsoever! There's little metal handrails between the seats, but not arms, they just raise a couple of inches. The seat has a certain width, the cushiony part, and then it extends a couple inches more on either side with just plastic, and then there's the rail. It's hard to explain. Anyway, I fit just fine there, the seat next door was occupied by a kid, but even a full-sized adult wouldn't have been a problem, I don't think. The seat belt wasn't hugely oversized like other places, but was definitely big enough. I did have a little problem buckling and releasing it, but I think it was because I was nervous. I stood up a slight bit, snapped it in place, and sat down all the way. Fine and comfortable (and a great ride).