Bruce, I e mailed you the FAA AC45-D2 info, but below is from AOPA with the same information.
Required Physical Attributes
For most fixed-wing aircraft, the marks must meet the following requirements:
Height: 12 inches
Width: two-thirds wide as they are high
The number "1" must be one-sixth as wide as high
Letters "M" and "W" may be as high as they are wide
Thickness: solid lines one-sixth as thick as the character is high
Spacing: no less than one-fourth of the character width
Uniformity: same height, width, thickness, and spacing on both sides of the aircraft.
Exceptions
Note: No person may operate an aircraft under the exceptions listed below if one or more of the following apply:
Operations in an ADIZ or DEWIZ described in FAR Part 99 unless temporarily has markings in accordance with FAR 45.21 and 45.23 through 45.33.
Operations in a foreign country unless that country consents to that operation.
Operations conducted under FAR parts 121, 133, 135, or 137.
The following aircraft may display marks at least 2 inches high:
Aircraft that had 2-inch markings displayed before November 1, 1981, and an aircraft manufactured after November 2, 1981, but before January 1, 1983, may display those marks until the aircraft has been repainted, restored, or changed.
Antique/classic aircraft: Aircraft built at least 30 years ago, or an aircraft issued an experimental certificate under FAR 21.191(d) or (g) for operation as an exhibition aircraft or as an amateur-built that has the same external configuration as an aircraft built at least 30 years ago may display marks at least 2 inches high.
The following aircraft may display marks at least 3 inches high:
Gliders, airships, spherical and non-spherical balloons, powered parachutes, and weight-shift-control aircraft.
Experimental exhibition, amateur-built, or a light sport aircraft under FAR 21.191(d), (g), and (i) when the maximum cruising speed of the aircraft does not exceed 180 knots CAS.
Other exceptions:
FAR 45.22 for exhibition, including motion picture or television production, or an airshow aircraft.
Note: If, due to the configuration of an aircraft, it is impossible for a person to mark it in accordance with FAR 45.21 and 45.23 through 45.33, he may apply to the FAA for a different marking procedure.
When 12-Inch Numbers Are REQUIRED
All aircraft penetrating an air defense identification zone (ADIZ) must have markings at least 12 inches high, at least temporarily. Using 2-inch-wide electrical tape is an acceptable method to temporarily display the aircraft's registration markings.
