Thats very inspiring! So you dont think 18 is to big of a wheel to offroad with. I live in the rocky north east. Most go with 16, but i dont know if the difference in size will dramatically decrease offroad comfort.
there is no wheel size too big for off-road
point is - you need as much rubber between the ground and the wheel
the more volume of air a tire has, the better for off-road
if you would compare two tires equal in diameter and compare 16" wheel to 18" wheel = the tire mounted on a 16" wheel has more volume = better for off-road - more comfort on-road (air in tires act as "secondary" springs)
for on-road performance (especially high speed and fast cornering) you want as little rubber as possible between road and wheel
please also note: all stock 4x4 tires are way too small for real off-road use
since 99.99% of all 4x4 are operated on pavement its the right choice from the manufacturer
if you ever want to leave pavement, you must get larger tires from a dependable manufacturer (not many to chose from)
larger tires have a myriad of advantages (need a new thread for that) and a few draw backs (expensive secondary modifications like lower gears)