Hey Matthew,
I'm also in the market for a new set of tires for the wife's '00 4Runner, but our OEM tire size is 265/70-16. Tirerack.com shows 3 other options, 275/60-17 or 285/55-18 or 265/50-20. I have seen a 4Runner with the 20" rims and I thought it looked sweet, but the wife didn't. Her car so she gets to choose. Are you looking into All-Season tires or Winter tires? If winter tires, I would stay with the OEM size. Just my 2 cents! Anyway, try tirerack.com it should give you all your options.