Expedition Everest is a very comfortable ride for larger people. Not only is it wide (even with a seat divider, which in many coasters makes for a tight fit for even some average sized people) but nice amount of leg room, for a coaster anyways. There are individual lap bars that also are very forgiving and nicely padded. It also has a head rest. It is not a lazy boy in terms of comfort but probably the closest you will get in a roller coaster seat.
I have only been to WDW once. I enjoy Disneyland more but this is the one ride I always miss on the west coast and probably one of the main things to draw me back to Orlando. I rode it 5 times in a half hour using the single rider line. I only stopped because I was developing a headache. The backwards part though is pretty intense, for a Disney ride anyways. Having your head firmly pressed against the head rest going backwards helps though.
If Big Thunder gets new trains as rumored, I hope the design is based on EE. This should be the gold standard for a roller coaster train.
Walt Disney World's Magic Kingdom
Disney's Animal Kingdom
Disney's Hollywood Studios
Disney California Adventure
Universal's Islands of Adventure
Universal Studios Florida
Universal Studios Hollywood