As per DPI. You can go 2 inches larger than the standard rims that came with your car. The size of the rim that came with your car is outlined on the placard just as you open the driver's side door.
Width I'm not sure. But I know that if you go larger you are suppose to maintain the same rolling diameter so that your speedo isn't out of sync. In that case, the width must be proportional to your increase.