Look up the revs/mile on your current tires, then find a size that matches within a percent or so. Same tire, tread, etc. as what's on the front.
I wouldn't do it, for a number of reasons, even if there is a tire that meets all the criteria, and you find wheels with the correct offsets and widths to match the tires.
Then again, I swapped out the 325/65R18s on my TRX for 295s (almost identical revs/mile) since that size matches the rim width much better, improving the handling and a couple of other things.
Obviously that sure doesn't look meatier, but to me it looks much better than stock since the tire width is now more correct for the wheels.