Here is how to calculate the size from Tirerack's pages:
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=7 You can sometimes find replacement drive gears for the speedo that fit internally in the tranny to compensate for up or down sizing of the diameter.
One thing that many people don't take into consideration is the torque change that you get when you up-size the diameter. You are changing the overall drive ratio with a larger diameter tire just as if you were going to a higher pitch prop or changing the gear ratio on your vehicle.
The result of a larger tire diameter is slower acceleration and a lower cruising RPM.
You also take away some of the aerodynamics and limit yourself to fewer tires but these are of lesser impact.