— The application you want will determine the right size of gravel to use. 3. What size is #4 gravel? #4 gravel measures about 1 – 2.5 inches in diameter (25 – 63.5 mm). 4. What size of gravel is best for a driveway? #3 Gravel is the best size for driveways since it provides a stable platform and proper drainage thanks to its medium-sized ...