AviationTire Air Law And Atc Procedures Annex 7 - Aircraft Nationality And Registration Marks Q 2 / 5

The height of the marks on the fuselage (or equivalent structure) and on the vertical tail surfaces of heavier than air aircraft shall be

at least between 20 centimetres and 40 centimetres
at least 40 centimetres
at least 30 centimetres
at least 20 centimetres