Here's an interesting read about the size of motorcycle mirrors for inspection in Pa. Not that anyone follows it but anywho.
175.148. Mirrors
(a) Condition of Mirrors - Mirrors shall be in safe operating condition as described in §175.160 (re lating to inspection procedure). (b) Rearview Mirrors - A motorcycle shall be equipped with at least one rearview mirror or similar device. If a certificate of exemption for a sun screening device or other material has been issued, rearview mirrors, each with a minimum reflective surface of 12 1/2 square inches for a flat mirror or 10 square inches for a convex mirror, shall be installed on both sides of a motorcycle. A vehicle for which a certificate of exemption has been issued for medical reasons may be equipped with only a left outside rear view mirror, unless originally equipped with an outside rearview mirror on both sides of the vehicle. (1) A mirror may not be cracked, broken or discolored. (2) A mirror shall hold adjustment. (3) A mirror shall provide an unobstructed view of the highway to the rear of the vehicle for distance of not less than 200 feet. (4) A mirror shall provide a minimum reflective sur face of 12 1/2 square inches for a flat mirror or 10 square inches for a convex mirror.
