I had 265 tires on a 7" wheel and that was pretty much all you could do. I know folks put on 285's on the stock wheels, but they pull the sidewalls in a lot and not recommended as Marty said.
My F150 has the 20" wheels that are 8.5" wide and have 275's on them. I could probably go to a 285 or even a 295 at most, but I'm not interested in doing that with this truck.
Without a level or 'lift' you might find 285's might rub on the front when turning lock to lock.. You can usually get away with trimming whatever it's rubbing on if it's plastic. If you start rubbing the fender itself, then you should raise it a bit.
Good luck!
Mitch