The smallest size of landscaping gravel contains pieces of crushed rock no larger than 3/8-inch. The smallest bits break down to a sand-like powder. When small gravel is compacted, the stone powder and small pieces form an almost solid base. Small gravel is useful in an area with sandy soil because it keeps the surface material from sinking. Avoid using small gravel if you have clay soil or an area with poor drainage, since underground water cannot escape, which eventually causes cracking and other surface damage.
For a concrete, paver or other type of paved driveway, medium gravel is the most common size used. Rocks range from 1/2- to 3/4-inch and come in irregular shapes. The pieces pack snugly together when tamped, but the different shapes leave crevices large enough for water to seep through. Medium gravel is also flexible so it can expand and contract when the ground freezes. Installing medium gravel below the frost line is one of the best ways to prevent frost-heave problems.
Measuring up to 1 1/2 inches, large gravel is typically used for gravel roads and driveways where a solid foundation is already built with small or medium gravel. Also shaped irregularly, the pieces compact together, but the spaces between stones are proportionally larger. It's good for drainage, but not stable enough to provide a sturdy foundation. Note that the shapes can have sharp edges, so it's not good to use for patios, walkways or any area where people walk barefoot.
Pea gravel is a round, smooth stone that comes in two basic sizes, small and large. Small pea gravel varies between 1/8- to 1/2-inch, while large pea gravel measures up to 1 1/4 inches. With a light brown to translucent color palette, it brings warm, neutral colors to a landscape. While attractive, pea gravel does not stabilize due to its round shape and should not be used as base material. If you prefer this look for a gravel driveway, install a solid gravel foundation with small gravel first to support the top layer of decorative stones.