My general rule is 31's will fit any scout but will rub in some situations, 33's will fit any scout with a 4" lift but will rub in some situations. The Scout wheelwells are only 32" long so anything bigger than 33's you need to lift it high enough to get the body above the tires, 6"+ to clear 35's, or trim the fenders to clear. If you're aggresively offroading it add the adjective "BADLY" in front of "RUB" above. If you never want to hear a tire kiss the sheetmetal, jump down one tire size. At that you still CAN rub them, but you have to work at it.
If it's a fullsize Pickup/Travelall, jump up one tire size. IE: 33's fit stock, 35's with 4" lift.
There are lots of exceptions to this, but for a basically stock truck with just a lift on it that's where most will fall. It really depends on how you build it, how you drive it, and how much rubbing you are willing to tolerate.