4 Inches. I have the old Blazer 2200 foglights that were amber, 55 watts, with glass lenses and are no longer being made. I actually had 2 sets of them over the last 12 years and 125,000 miles since the car was new-stones, chips, being sandblasted over the years.

Does anyone know of a quality glass 4 inch fog light you can buy now?

I found these:

105959 Extreme Dimensions Fog Lights- Large (4-inch diameter) for 97-03 Pontiac Grand_Prix at Andy's Auto Sport