3/8" diameter hail probably won’t damage your car, even if you drive in it, but hail can suddenly get bigger. Hailstones that get much larger than golf ball size are getting close to glass-smashing proportions, including possibly breaking out your back window or even your windshield. Like any kind of inclement weather, if you can avoid driving in it, do so. For larger hail, definitely try to find shelter for your car. If a hailstone smashes a window in your car, you could be injured either by glass or by the hail which suddenly has a way into your car.