What is the stock size? Generally pickups have a lot of clearance so I would say that you can go up at least two sizes and be OK. Example - 265/70/17 can be upgraded to 285/70/17 without issues on my Ford and I could probably still go one more size up.
Good tire shops like American Tire have a guide on how big a tire will fit without modifications. If you go wider, make sure there are no parts that the tire will rub against and cut the tire. If you go bigger, not a bad idea to add stiffer shocks to stop a lot of body movement. Is it for looks, or for economy? Also if you see other trucks like your model with larger tires, ask them about it and see if it will be worth the move Full-of-Regrets is correct. Two sizes up may be possible.
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