Not really so easy - I looked into it, and the only logical combination would be a 19" wheel with a larger diameter tire. The larger tire mounted on the front would require extremely careful driving - it's 1" more in diameter, which would mean potential problems with the tire rubbing the fender on anything except a very shallow turn.
You can't run a front wheel and tire on the rear because of the smaller diameter - the Torsen locking rear end won't be happy - potential for serious differential problems. I will probably have the wheels off to paint the calipers this fall, and I'll try a front wheel on the rear. If it has enough clearance not to rub on any critical parts it wouldn't require a spacer.
I must admit I was a little nervous driving to the west coast from North Carolina with no spare, but no problems. Next major trip will be to Vegas and San Francisco, and back along the coastal highway, with fingers crossed. (Haven't told my wife yet that there is no spare - don't want to ruin the joy of carefree driving.)