Part 45.29 Size of marks.
. . .
(e)
Spacing. The space between each character may not be less than one-fourth of the character width.
(f) If either one of the surfaces authorized for displaying required marks under ?45.25 is large enough for display of marks meeting the size requirements of this section and the other is not, full-size marks shall be placed on the larger surface. If neither surface is large enough for full-size marks, marks as large as practicable shall be displayed on the larger of the two surfaces. If any surface authorized to be marked by ?45.27 is not large enough for full-size marks, marks as large as practicable shall be placed on the largest of the authorized surfaces.
(g)
Uniformity. The marks required by this part for fixed-wing aircraft must have the same height, width, thickness, and spacing on both sides of the aircraft.