I'm around your size (last time I checked anyway) and I don't have any issues at Alton Towers. The only 'issue' I had was on Rita as a few seats have a shorter secondary seatbelt restraint thingy that others. I rode it back to back, first time was fine and the second time I didn't fit. They ended up bumping me to the front row that time and I was fine
How tall are you? I'm 1.93M (6 foot 4), on a few rides, Smiler and Nemesis especially, the OTSR is only a few CMs from my shoulders giving me some slight bruising after a few rides. Wearing a backpack was slightly painful for a few days!