Kevin:
I've heard about folks running 18" wheels, but 17" seems to be the sweet spot, as our suspensions were designed to work with a fair bit of tire flex. Key is making sure that the diameter of the tire you're using is comparable to (or maybe even a bit smaller than) the diameter of the stock tires -- otherwise, you're going to lose some power spinning the larger diameter tire
Check out the bottom of this thread for some good info
http://forums.pelicanparts.com/porsche-911-technical-forum/1093626-911sc-fuchs-changing-6-7-911-7-8-951-a.html