Welcome to the site! Really, it's all up to you and how big you want to go. First of all, do you have a 4x2 or 4x4? If you have a 4x4, I think the best looking combo with a 2.5 inch leveling kit is a 325/60-18 (33x13.00) tire on an 18x9 inch wheel with 5.5 inches of backspacing. There are a lot of variables when it comes to fitment, but as a general rule on the new 4x4's, you can fit 33's with a leveling kit and aftermarket wheels or 35's with a leveling kit and the OEM wheels.
2005 Expy, 305/45-22 Falkens on 22x9.5 Eagle 026s.
2011 Jeep Wrangler Unlimited, 2.5" lift, 35" Trail Grapplers on 17x9 Eagle 197s.
2012 Ram 2500 crewcab diesel. 5" RCX lift, 355/60-20 Nitto Terra Grapplers on 20x9 KMC XD Spys.