I was 300 pounds last year when we went. I had no problems fitting in any of the rides. Here are a few things I did find out over the years though...
Dinosaur is uncomfortable on my hips because of my size. It actually hurts riding that ride when you get jostled around, but I still do it because I love the experience
I always request the front row on coasters, etc. they have more leg room.
Primevil Whirl is kind of uncomfortable because of my size but once again I still ride it
I even fit in the ride(I temporarily lost the name) in Tomorrowland the spaceships that you take the elevator to get to and they spin around really high. It was just extremely hard for me to get out of.
When riding a ride with a seat belt, I always pull out my seatbelt before sitting down and buckle or else it is hard to get to once I am seated.
I don't get on a horse on the carousel, I did that once and when the ride stopped my horse was way up in the air and...

I still laugh just thinking about the sight...
I guess the greatest tip I can give you is to ask for the front rows, esp. in rides like SM and ToT and RnRC.
Check out allears.net WDW at large. It really helped me
