FAA Regulations

The following is a summary of the FAA regulations related to registration numbers on aircraft.
(Read the actual excerpt from 14 CFR 45 by clicking here)

REGISTRATION MARKING SIZE

Aircraft Type	Minimum Registration Size
PRODUCTION AIRCRAFT
Aircraft less than 30 years old	12″
Aircraft 50 year or older	2″
EXPERIMENTAL AIRCRAFT
Up to 180 Kts C.A.S.	3″
Faster than 180 Kts C.A.S.	12″

REGISTRATION MARKINGS PLACEMENT

Aircraft/Marking Type	Minimum Registration Size
Production or Experimental Aircraft — 12″ or 2″ markings	Horizontally on the: Tail Fuselage between the trailing edge of the Wing and the leading edge of the Horizontal Stabilizer Jet Engine Nacelles
Rotorcraft 12″ Markings	Horizontally on the: Fuselage CabinTail Boom
Experimental 3″ Markings	Horizontally on the: Tail Fuselage between the trailing edge of the Wing and the leading edge of the Horizontal Stabilizer.