I went with a 15x6 in the front but had to use a 195/60R15 tire to have clearance. The fronts are 7.9" wide within a 8.5" space with a diameter of 24.2". The rears are 15x8 with 225/60R15 tires. The rears are 10.5" wide within a 11" space with a diameter of 25.8. I like this combo because it gives just the right amount of rake. The biggest problem I had was the backspacing on the rims as they both had to be maxed to fit.
Here is a link to a tire chart that will give you width and diameter. It is the same place that made my rims for me (Vintage V). They really look great on my car.
http://www.team3wheels.com/tech_specs_tire_chart.html