[WFBowen]

The original overall diameter is 25.2” so any tire within a half inch of this will look like it belongs on the car. Lower than this will lower the car, start to effect gearing and speedometer readings and visually create a visual open space in the top of the wheel well.
As long as you stay to an available variant of the original 185/70r15 all will be well.
JMO,
Bill