As per transport Canada, here....
http://www.tc.gc.ca/eng/civilaviation/regserv/cars/part2-standards-222-1005.htm
Specifications for the Letters in the Marks Displayed on Aircraft
(2) The letters in the marks displayed on an aircraft shall meet the following specifications:
(a) be of equal height;
(b) subject to subsections (c) and (d), the height of each letter in the marks displayed on a heavier-than-air aircraft shall be not less than 15 cm (5.9 inches);
(c) the height of each letter in the marks displayed on the bottom surface of a wing of a heavier-than-air aircraft shall be not less than 50 cm (19.68 inches);
(d) the height of each letter in the marks displayed on the bottom surface of the fuselage or cabin of a rotorcraft shall be the lesser of
(i) 50 cm (19.68 inches), and
(ii) four fifths of the width of the fuselage or cabin;
(e) the height of each letter in the marks displayed on a lighter-than-air aircraft shall be not less than 50 cm (19.68 inches);
(f) the width of each letter in the marks displayed on an aircraft, other than the letters "I", "M" and "W", shall be two thirds of the letter's height;
(g) the width of the letter "I" in the marks displayed on an aircraft shall be one sixth of the letter's height;
(h) the width of the letter "M" or "W" in the marks displayed on an aircraft shall not exceed the letter's height;
(i) the length of a hyphen in the marks displayed on an aircraft shall be two thirds of the height of any letter in the marks;
(j) the thickness of the lines of a letter or hyphen in the marks displayed on an aircraft shall be one sixth of the height of any letter in the marks;
(k) the letters in the marks displayed on an aircraft shall be displayed adjacent to each other in a series;
(l) adjacent letters in the marks displayed on an aircraft shall be separated by a space that is not less than one quarter of the width of the letter "C" in the marks;
(m) a letter in the marks displayed on an aircraft that is adjacent to a hyphen shall be separated from the hyphen by a space that is not less than one quarter of the width of the letter "C" in the marks;
Marks Displayed at an Angle
where the letters of marks are displayed at an angle,
(i) the letters are displayed at an angle of not more than plus or minus 35 degrees to the perpendicular of their base,
(ii) the angle of each letter and of both sides of the hyphen comprising the marks are the same,
(iii) the height of the letters is measured perpendicular from the base line of the letters, and
(iv) the width of the letters, the spacing between the letters and the width of the hyphen are measured parallel to the base line and between the lines that define the outside edges of each letter and the hyphen;