Well the answer depends on if you get a 2wd or a 4wd. There was no equivalent to the Edge package in 98-99, so there are no 2wd trucks w/ the Torsion bar suspension. (The 2000 "Trailhead" was the first precursor to the Edge package.) On a 4x4, 31x10.5 is a good all-around choice for offroading. 32x11.5 is just about the biggest you can cram onto one without major rubbing. If you get a 2wd, I'm not even sure a 31x10.5 will fit without rubbing.
Also, there are no torsion bars on the back of the truck. The only "cheap" way to change the rear ride height is by getting larger lifting blocks, but then you really need to shim to keep the pinion angle within spec, and it gets to be a big headache relatively fast for a first-timer.
2003 Edge Reg Cab 2x4 3.0L 5M 8.8LS - Red
1991 Miata - British Racing Green Special Edition (3716 of 4000)