I wouldn't go bigger than 20". The cost of tires will be enough as it is and you will have a better range of sizes to choose from.
If it was my truck I'd stay at 18" rims because I'm not a fan of big rims.
X2 no bigger than 20's.
2008 F-150 XLT Scab 4x4
Mods> MBRP Cat-Back SISO, ProComp 18" 7032, 295/65/18 Wrangler Duratrac, 2" AS level, Saleen grille, reverse lights, fogs in lower bumper, PTM mirror caps, door handle Keypad, under rear seat storage, 09+ badges, Kenwood 6"x8"3 way speakers.