I'm not a plus size person, but I had a guy that was about 6'3" and 325+ pounds next to me on the Hulk back in 2003. Two people had to push down on the upper body restraint to get it latched. It was a little embarassing for him since the first girl indicated he wasn't going to fit which held up the ride, but someone else came along and they got the belt latched and we were off.
I think it's all going to be based on how big your upper body is. The staff that was running the Hulk seem to handle the situation very well.
We frequently go to Cedar Point in Ohio and starting last year they implemented some pretty strict weight policies and at the beginning of last year the staff wasn't handling it very well IMHO. We went once last year and every ride seemed to have people that were not allowed on because of the new size restrictions (very upset people). I heard later in the year most of the major rides at seats at the beginning of the ride line were people could test fit themselves into the seats before waiting in line and finding out they wouldn't be allowed to ride.
I believe IOA has test fit seats at the entrance to each major ride. If you can fit into the test seats you will be good to go.
As for some of the kids oriented rides, I about 155 and I barely fit in some of the Suess Landing rides.
Your going to have fun either way, have a good trip!