My '01 and old '02 Explorer have 255/70r16 which is a 30" tire like Ken said. You could probably fit a 245/75r16 tire with no problem.
I also agree with Ken on ball joint failure. My '02 didn't have any problems, but I have had a lot with the '01.
Also and '03 had the IRS rear which is junk, So not lift kit availible unless you get a body lift. I ate up bearing/hub assemblies constantly in the rear, and that was with street driving and stock size tires. Not sure if he would want to put larger tires on it and stress it more.