For larger guests concerned about fitting in this ride - I will say that I had problems with HP Forbidden Journey when it first opened. But I had no problems fitting in the Avatar - Flight of Passage ride.
It was restrictive in that once the back & leg restraints were locked in. But it was like that for everyone. It has to lock your lower body in place to stay on the ride.
The difference between Harry Potter & Avatar size limitations is where the restraints hit. I had issues with HP because of my chest. Even though I do have large hips and calves, I had no problems with the Avatar ride.