The stock diameter for a wagon is about 28 inches. Any substantial deviation from that will cause about 3 mph per 1" of tire diameter (larger diameter will cause the speedometer to be off slow). You can use any combination of wheel/tire that will give you the 28" diameter. For instance, a 17" wheel with a 255-60-17 tire will be about 28" in diameter. Tires are light, wheels are heavy, and a taller sidewall will give you a smoother ride. Make sure you have the stock +6 mm offset, or you may run into rubbing issues. An 8" wide rim on the front, with the +6 mm offset, will be fairly close to tubbing the outer tierod end. At 8.5" wide with +6 mm offset, you have about 1/16" of clearance at the tierod end, and must use tape-a-weights to balance the wheels.