The majority of foam rollers are 5 or 6 inches in diameter and 36 inches in length. This size is recommended for your back because they are long enough to span your entire back when placed perpendicular to your spine. This size is a bit more stable than shorter rollers when working on quads, hamstrings and other large muscle groups.
Other foam rollers are 3- or 4-inch diameter and 4 to 24 inches in length. They help to target a deeper massage for specific smaller muscle groups such as arms and calves.