Assuming you keep your stock wheels and no leveling, the LT285/65R18 (32.7 inches tall and 11.5 inches wide on a 8.5 inch rim) would be pushing it without trimming. A safe bet has been the LT275/65R18 (32.1 inches tall and 11 inches wide on an 8-inch rim). Some will say they are running LT305/65R18 (33.7 inches tall and 12 inches wide on 8.5-inch rim) but for most requires some trimming, is getting close to the upper control arm, and not giving you much for articulation if want to go offroad. But every truck is slightly different; I would dry fit a 285 and go up/down from there.