I dont know specifics about Larger seats on the rides at US/IOA, however when we were there last week we encountered two not so happy instances for "hulk" sized guests.
My teenaged dds rode Mummy and after getting off told me the story of a family that appeared to be embarrassed/upset because the "dad" didnt fit in the seat of the ride so they all got up and got off. I didnt ride the Mummy so I dont even know what the seats looked like. I thought it sounded a little strange and thought that maybe they were confused.
However, strange enough later in the week, dh and I witnessed something similar ourselves. We decided to ride DD without the kiddos

We were next up to load and watching the group in front of us. I am not sure how to explain this, but when loading you kind of "step sideways" into the row of two seats to get in your seat. I think a really skinny person could walk facing forward, but me, averaged sized, I walked kind of sideways to the seat. There was a lady who physically couldnt get into the car to get into her seat. She was in the very back row (I am not sure if these are larger seats or normal sized). She was a very pooh sized person. They actually put on the "technical difficulties" announcement and I have no idea what they said to her (they were being quiet and discreet) but she was walked to the exit door (time elapsed about three minutes). I didnt pay attention to the initial loading so I dont know if she was alone or with other people. I dont even know if she was trying to get in a large seat or a regular one. Also, be prepared if you wear flip flops you will have to sit on them for the ride
Sorry I dont know the details or specific ride/size restriction requirements, but hope that you find out ahead of time and are able to enjoy all the rides at Universal. It was our first trip, and we had a GREAT time.
One great thing we loved about Universal were the free lockers located outside the thrill rides for storing your stuff (two hours free).