I agree, both hubby and I are very plus sized and it's the one place where we can ride almost everything. Depending on how tall you are you may have to adjust where in the ride you sit. My hubby is 6'4" and the only thing we have to do is sit in the front of some cars (not the first row, just the front seat of any car) for example Rockin Roller coaster and Space Mountain. Cuz there is no seat in front of you there is more leg room and so he is able to ride those with little to no discomfort.
Our "trick" for Thunder Mountain is to say we're single riders. While we both fit in a row together ok, but we found that sitting in row to ourselves was more enjoyable. (Personally, I slide side-to-side a bit going around the turns and I find more enjoyment in feeling the ride more that way.

) We never thought about it until a CM asked us if we wanted to split up, so we tried it and that's how we ride now.
I will caveat we have not done Mission Space in years so I can't remember how the seats were there. The Sum of All Thrills ride in Innovations Epcot is probably the only ride I would doubt. Even if hubby wasn't so Pooh sized he was probably too tall to fit (the CM looked at him and said, 'i don't think it'll work' and sure enough she was right

) and even though I'm average (5'6" ish) my thighs were too big for the shoulder harness to lock all the way. So that's the one ride we wanted to do but couldn't. Other than that, I recall pretty much everything was accommodating, unlike Universal. Universal we had lots of trouble with the rides, even with the bigger seats.
I second allears.net they have a good page outlining the different rides with user experiences.