There are no weight restrictions on any attraction that I know of.
In fact the only weight restriction for anything is for anything that uses a Segway for a tour (250 lbs).
You should read this.
http://allears.net/tp/ridsiz.htm
And the AK Wildlife Trek Tour (the one that includes a rope bridge and zipline) is, I think, 315 lbs., and they will weigh you.
No rides at WDW, including the waterpark, have weight limits. I'm 5'10", about 300 lbs., wear size 3XL shirts, and could fit on every ride at WDW except for the two minor roller coaster simulators, Sum of All Thrills at Epcot and Cyberspace Mountain at DisneyQuest. Two big people may have trouble sitting together on Kali River Rapids (but will be fine separated so they don't have to share a seatbelt) and big people may have trouble getting into/out of Space Mountain.
Rides with restraining lap bars (as opposed to bars that just keep you from walking out of the car): Splash Mountain, BTMRR, Space Mountain, Expedition Everest
Rides with overhead restraints: Mission: Space, RnRC
Rides with seatbelts: Test Track, Soarin', Star Tours, ToT, Kali River Rapids, Dinosaur
All other rides shouldn't be a problem.