This is long but the jest of it is if the A/C was built 30 yrs ago or is experimental it can use 2" numbers, otherwise they are supposed to be 12".
§45.21** General.
(a) Except as provided in §45.22, no person may operate a U.S.-registered aircraft unless that aircraft displays nationality and registration marks in accordance with the requirements of this section and §§45.23 through 45.33.
(b) Unless otherwise authorized by the Administrator, no person may place on any aircraft a design, mark, or symbol that modifies or confuses the nationality and registration marks.
(c) Aircraft nationality and registration marks must --
(1) Except as provided in paragraph (d) of this section, be painted on the aircraft or affixed by any other means insuring a similar degree of permanence;
(2) Have no ornamentation;
(3) Contrast in color with the background; and
(4) Be legible.
(d) The aircraft nationality and registration marks may be affixed to an aircraft with readily removable material if --
(1) It is intended for immediate delivery to a foreign purchaser;
(2) It is bearing a temporary registration number; or
(3) It is marked temporarily to meet the requirements of §45.22(c)(1) or §45.29(h) of this part, or both.
[Doc. No. 8093, Amdt. 45-5, 33 FR 450, Jan 12, 1968, as amended by Amdt. 45-17, 52 FR 34102, Sept. 9, 1987]
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§45.22** Exhibition, antique, and other aircraft: Special rules.
(a) When display of aircraft nationality and registration marks in accordance with §§45.21 and 45.23 through 45.33 would be inconsistent with exhibition of that aircraft, a U.S.-registered aircraft may be operated without displaying those marks anywhere on the aircraft if:
(1) It is operated for the purpose of exhibition, including a motion picture or television production, or an airshow;
(2) Except for practice and test fights necessary for exhibition purposes, it is operated only at the location of the exhibition, between the exhibition locations, and between those locations and the base of operations of the aircraft; and
(3) For each flight in the United States:
(i) It is operated with the prior approval of the Flight Standards District Office, in the case of a flight within the lateral boundaries of the surface areas of Class B, Class C, Class D, or Class E airspace designated for the takeoff airport, or within 4.4 nautical miles of that airport if it is within Class G airspace; or
(ii) It is operated under a flight plan filed under either §91.153 or §91.169 of this chapter describing the marks it displays, in the case of any other flight.
(b) A small U.S.-registered aircraft built at least 30 years ago or a U.S.-registered aircraft for which an experimental certificate has been issued under §21.191(d) or 21.191(g) for operation as an exhibition aircraft or as an amateur-built aircraft and which has the same external configuration as an aircraft built at least 30 years ago may be operated without displaying marks in accordance with §§45.21 and 45.23 through 45.33 if:
(1) It displays in accordance with §45.21(c) marks at least 2 inches high on each side of the fuselage or vertical tail surface consisting of the Roman capital letter "N" followed by:
(i) The U.S. registration number of the aircraft; or
(ii) The symbol appropriate to the airworthiness certificate of the aircraft ("C", standard; "R", restricted; "L", limited; or "X", experimental) followed by the U.S. registration number of the aircraft; and
(2) It displays no other mark that begins with the letter "N" anywhere on the aircraft, unless it is the same mark that is displayed under paragraph (b)(1) of this section.
(c) No person may operate an aircraft under paragraph (a) or (b) of this section --
(1) In an ADIZ or DEWIZ described in Part 99 of this chapter unless it temporarily bears marks in accordance with §§45.21 and 45.23 through 45.33;
(2) In a foreign country unless that country consents to that operation; or
(3) In any operation conducted under Part 121, 133, 135, or 137 of this chapter.
(d) If, due to the configuration of an aircraft, it is impossible for a person to mark it in accordance with §§45.21 and 45.23 through 45.33, he may apply to the Administrator for a different marking procedure.
[Doc. No. 8093, Amdt. 45-5, 33 FR 450, Jan. 12, 1968, as amended by Amdt. 45-13, 46 FR 48603, Oct. 1, 1981; Amdt. 45-19, 54 FR 39291, Sept. 25, 1989; Amdt. 45-18, 54 FR 34330, Aug. 18, 1989; Amdt. 45-21, 56 FR 65653, Dec. 17, 1991; Amdt. 45-22, 66 FR 21066, Apr. 27, 2001]
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§45.23** Display of marks; general.
(a) Each operator of an aircraft shall display on that aircraft marks consisting of the Roman capital letter "N" (denoting United States registration) followed by the registration number of the aircraft. Each suffix letter used in the marks displayed must also be a Roman capital letter.
(b) When marks that include only the Roman capital letter "N" and the registration number are displayed on limited or restricted category aircraft or experimental or provisionally certificated aircraft, the operator shall also display on that aircraft near each entrance to the cabin or cockpit, in letters not less than 2 inches nor more than 6 inches in height, the words "limited," "restricted," "experimental," or "provisional airworthiness," as the case may be.
[Doc. No. 8093, Amdt. 45-5, 33 FR 450, Jan. 12, 1968, as amended by Amdt. 45-9, 42 FR 41102, Aug. 15, 1977]
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§45.25** Location of marks on fixed-wing aircraft.
(a) The operator of a fixed-wing aircraft shall display the required marks on either the vertical tail surfaces or the sides of the fuselage, except as provided in §45.29(f).
(b) The marks required by paragraph (a) of this section shall be displayed as follows:
(1) If displayed on the vertical tail surfaces, horizontally on both surfaces, horizontally on both surfaces of a single vertical tail or on the outer surfaces of a multivertical tail. However, on aircraft on which marks at least 3 inches high may be displayed in accordance with §45.29(b)(1), the marks may be displayed vertically on the vertical tail surfaces.
(2) If displayed on the fuselage surfaces, horizontally on both sides of the fuselage between the trailing edge of the wing and the leading edge of the horizontal stabilizer. However, if engine pods or other appurtenances are located in this area and are an integral part of the fuselage side surfaces, the operator may place the marks on those pods or appurtenances.