My husband is 6'4 and 300+ lbs. He rides comfortably in all of the disney rides but I am tiny 5'0 and 100 lbs, so we ride fine together in most everything.
So, let's say that he went to WDW with one of his brothers...in that case these are the rides that they would not be able to ride together:
-Teacups (tight around the legs & knees, not easy to get into)
-Astro Orbiter (we couldn't ride this together, it is tight in the legs/knees area)
Maybe:
-Big Thunder Mountain
-EE
-Peter Pan
-Haunted Mansion
Rides that I can think of could be cramped b/c of the size of the seat are Dinosaur, Tower of Terror, Rockin Roller Coaster and Mission Space, but my husband has never had trouble with any of these.
For rides that typically fit 3-4 people, just let the CM know that you need your own car - Test Track & Primeval Whirl can be really uncomfortable with 3 plus sized folks, but with just 2 in the car you should be fine
There are lots of rides that have seats made for two tiny people, so one of you could go in the front while the other sits in the back - Snow White, Winnie the Pooh, etc.
Also, for rides like Its a Small World, Living w/ the Land or Pirates of the Caribbean, ask to sit in the front of the boat b/c there's more legroom there.
Above all, don't stress about it too much, the CM's are very good at accommodating everyone.
