The ICAO Standards for registration marks re contained in Annex 7
3. LOCATION OF NATIONALITY, COMMON AND REGISTRATION MARKS
3.1 General
The nationality or common mark and registration mark shall be painted on the aircraft or shall be affixed by any other means ensuring a similar degree of permanence. The marks shall be kept clean and visible at all times.
3.3 Heavier-than-air aircraft
3.3.1 Wings. On heavier-than-air aircraft, the marks shall appear once on the lower surface of the wing structure. They shall be located on the left half of the lower surface of the wing structure unless they extend across the whole of the lower surface of the wing structure. So far as is possible, the marks shall be located equidistant from the leading and trailing edges of the wings. The tops of the letters and numbers shall be toward the leading edge of the wing.
4. MEASUREMENTS OF NATIONALITY, COMMON AND REGISTRATION MARKS
The letters and numbers in each separate group of marks shall be of equal height.
4.2 Heavier-than-air aircraft
4.2.1 Wings. The height of the marks on the wings of heavier-than-air aircraft shall be at least 50 centimetres.
4.2.2 Fuselage (or equivalent structure) and vertical tail surfaces. 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 30 centimetres.
5. TYPE OF CHARACTERS FOR NATIONALITY, COMMON AND REGISTRATION MARKS
5.1 The letters shall be capital letters in Roman characters without ornamentation. Numbers shall be Arabic numbers without ornamentation.
5.2 The width of each character (except the letter I and the number 1) and the length of hyphens shall be two-thirds of the height of a character.
5.3 The characters and hyphens shall be formed by solid lines and shall be of a colour contrasting clearly with the background. The thickness of the lines shall be one-sixth of the height of a character.
5.4 Each character shall be separated from that which it immediately precedes or follows, by a space of not less than one-quarter of a character width. A hyphen shall be regarded as a character for this purpose.
No mention of why the marks have to be these particular sizes so I guess you'd need to ask an optician about the capabilities of the human eye and do a bit of geometry.
In practice there's lots of evidence that even those who are familiar with aircraft and the way they are operated have difficulty estimating the height of an aircraft overhead.