It depends on many factors. Some of them being:
-What is vehicles purpose?
-Where will it be driven?
-Towing anything?
-How much power do you have?
-How much does MPG matter?
-Can you afford to re-gear?
-Can you afford the larger tires?
My dodge has 40x15.5R20s on it, and they are to large for my trucks current usage. Getting into the cost factor, the tires are $620 each, and rims are around $350 each. So buying two spare tires driving to Alaska really really hurts. My MPG dropped from 17 to 14 with the 8" lift and tires. And no, I havent re-geared yet as I dont have the money or decided on tire size. I had to do cross-over steering, lengthen my drivelines, re-enforce my steering box and move it, and modify my front cross-member. Not to mention lift the truck 8". Now the truck cant tow very well up hills due to the larger tire size without re-gearing, and sheer weight. Each tire/rim combo weighs over 125 lbs!
I think a tire in a 35-37" range will work best for me. The tire weight is cut in half for one! The cost is also dramatically less. Finding a spare tire in the middle of no-where is easy for 35s, and very very hard for 40s. 35s allow me to go 90% of the places I can with the 40s, and get better mileage to and from there.:wings:
So it all depends on what you are willing to spend, and what your plans are.