Hey Mike,
One thing that is great about Disney is they think of this stuff. My brother-in-law is a round dude. We both wanted to ride Rock n Roller Coaster. He was concerned that he might not be able to ride ... He went to one of the Cast Members in the fast pass line and asked if he might have a problem. The cast memeber took us both back stage, to a sample car that was the same size as the one used on the ride. After sitting in the car, he realized it would be no problem. This was nice because it was descret. The cast member then took us both to the front of the line and we got right on. After we got off, he gave us fast passes to get back on the ride to experience the entire ride, que and all. This was so cool. Id recommend this to your family. And for the record, he should have no problem riding Rock n Roller Coaster.