stock rims are 7.5" wide. Tire manufacturer lists minimum width as 7.5" for 285/70/17 or 295/70/17. Many have larger on stock rims, but I prefer staying within recommended ranges listed by brand. Some tire shops may also not mount them if too wide for recommended rim width.
Definitely not locked in at stock size. Anything with a 285 or 295 width will fit on a 7.5 wide wheel. If you want wider an aftermarket wider wheel is recommended. That being said people do put 315 wide or 12.5 wide on stock wheels you just may get funny wear due to the thinner wheels or have run lower psi to get even wear.