I'm a big dude, and I've had tons of problems at amusement parks fitting on rides. (I waited for 3 hours to ride a roller coaster at a Six Flaggs only to find out I was too big!!!)
I've never had a problem at any of the Disney parks....
Magic Kingdom...no problems.....Space Mountain & Big Thunder Mt. have bars that come back to you, but they stop at your tummy. They don't have to click in very far, at all! (Same with the Goofy coaster)
Epcot...I can't speak for Mission Space because I passed on that... but Test Track is just regular car seat belts, and the Soarin' in CA I had no problem with...I assume the FL version is the same!
MGM- I passed on Rock n Roller coaster for many years, because I was sure I wouldn't be able to ride, last year I asked a Cast Memeber if I could ride and she said I wouldn't have a problem...and she was right. I sat in the front car and it felt very good to be on a good ol' roller coaster again!
Tower of Terror- the bar digs into my gut a little bit...but nothing major, I've been on it several times!
Animal Kingdom - the Primeval Whirl was a tight fit...but that was also because there were four people in my car!
In
Disneyland I was even able to squeeze myself into one of the cage cars on Casey Jr's Circus Train!
You'll have a great time!