The U.S. Postal Service stipulates that small mailboxes should be approximately 5 inches wide, 18.5 inches long and 6 inches tall, and medium mailboxes should be 6 inches wide, 19 inches long and 7 inches tall. Large mailboxes are 8 inches wide, 22.5 inches long and 11.5 inches tall.
The domestic mail manual sets the minimum interior dimensions for curbside mailboxes. The manual requires owners to position mailboxes about 6 to 8 inches from the curb and about 42 to 46 inches above the ground. USPS does not govern wall-mount mailboxes. Check local regulations, and notify the local mail carrier before installing these boxes.