All ears.net has a great section titled WDW at Large with lots of info about the rides, ride vehicles, potential problems and tips. The quick version is - WDW is very size-friendly and you will have much better luck there than just about any other amusement park. A lot depends on your height and where you carry your weight and you'll find lots of specific info there.
My dad was 6'3" and in the process of losing lots of weight when we last went (down to 325 from over 400) and I'm so glad he looked at that site, or else he might have been afraid to go. A few ride vehicles were more tricky because you're down low and have to hoist yourself out, rather than so tight you can't fit. I'm heavy (carry it all over - chest, belly, legs) but 5'8". Both dad and I had our concerns, but rode every single ride without troubles. We did not do Sum of all Thrills (don't do motion simulators). Rides with a seat belt, like Test Track are easiest if you pull the belt all the way out prior to sitting. Rides with a U-bar like Rockin Roller Coaster don't go down as far on me as on others, so it looked funny sitting by my 10year old daughter and we couldn't really look at each other because my higher bar blocked our view. I can't remember if Big Thunder Mountain has a lap bar that you share with your seat partner, which probably means it won't come down low on their lap seated beside a bigger person.
Try searching these forums for the phrase "pooh-sized", as that's a common Disney euphemism for larger guests.
