dnvrdrvr - I posted this thread in July of 2010 and obviously nobody took the bait.
Bigger has been proven time and time again to be better when it comes to autocross, assuming the wheels you're putting the big tires on actually do something and you're not making the car 6" wider. Some FWD cars, where the rear wheels are basically just there to keep the trunk from dragging on the pavement, go with a narrow rear tire to heat it up faster and whatnot, but even on super narrow wheels the wider tires wear more consistently and are more consistent run to run along with more lateral grip, partially as a consequence of the extra mass. There are downsides but they're almost always outweighed by the upsides. The difference between the 295 Hoosier and the 315 hoosier is 3lbs per tire, which is noise and on the rear of the car probably only helps traction at these low speeds, remember we're in 2nd gear here and we all know this car will go sideways with the twitch of a toe in 2nd when you're already cornering at the limit. It comes down to weight of the car per mm of tire width as a rough indicator as how successful the car can be, and if you can give up 12 lbs of weight for 60mm of tire it's generally a good trade. Gearing becomes an issue too, you don't want to put 345/35-18's on the car just because they're the biggest because you might force yourself to go down to 1st in too many elements, though this car has pretty good gearing and power delivery for autocross.
Some people in RWD's stick with the narrower front tire for better steering response, scrub radius, clearance, width (narrower overall width of the car is better in autocross) but those are usually on stiff cars that can get good camber. Even though the front suspension on this car is worlds better than the E46/E36 design it's far from ideal. Another plus of starting with the biggest tire possible is you can always go smaller and you know where the limits are.