My DH is large (he was about 370lbs on our trip last year, he has since lost 30 lbs

) and I must say how wonderfully accomodating everything at Disney was as far as size, and handicapped issues. So unlike most other parks we've been too. I have told several people I know about how pleased we were. We never had a problem with anything, but there were a few things we were prepared for...
1. Kali River Rapids, discreetly as the ride operator if you can sit by yourself. You don't have to say why, they'll get it

That way they won't automatically sit someone next to you.
2. Eating at the Sci-Fi. Those litle cars are tiny. Ask for a table, they are still car shaped, but with more room. Put it on your PS if possible. Remind them when you get there. Here is a link to a pic of DH and I at a table:
http://pages.sbcglobal.net/tonyandgiovanna/disney/scifi.jpg
Those were the only 2 issues were encountered the whole week. At first DH was nervous about the water park and not being "slim and sexy" but, it's not like going to Daytona Beach for Spring break. The water parks are filled with "pooh sized" middle aged parents having a ball with thier kids. So, if you were hesitant about that, don't! My DH had a blast at Blizzard Beach, and didn't feel self conscious at all.
Edited to add: We were worried about Space Mountain and Rock'n'Roller coaster. He was able to do both (though Space Mountain was a bit of a squeeze, but doable). You can check out the link above and compare his size to yours.