15x6 wheels with 195/50 tires is perfect for the smart IMHO - nice stance, meaty tires, but not too much. Anything wider up front, and I think it would look overdone. This setup is flush at the top of the arch. The 195/50 plays well with all the smart systems being a very, very close match and is readily available from multiple tire manufacturers in all-season, summer performance, etc.
I'm getting ready to do my first 4 tire rotation at 5K miles on this setup and it has performed flawlessly on the smart in various conditions (road trips, snow, daily commute, twisty back roads, etc.) and improves the ride.