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This really depends on what you are using the truck for and really is only limited by the front tires.
295/60R20 is one I have heard. Which would be 295/70R18.
I would guess right about 315 would be the max.
If you stay on pavement you can go to the big side...if you go offroad a little smaller.
And then, there is, the sawsall.
Good luck