The dimensions for oval stickers are stipulated within the UNECE Convention on Road Traffic, of 8 November 1968.
Annex 3 states (in part):
� The distinguishing sign... shall consist of one to three letters in capital Latin characters. The letters shall have a height of at least 0.08 m and their strokes a width of at least 0.01 m. The letters shall be painted in black on a white ground having the shape of an ellipse with the major axis horizontal.
� When the distinguishing sign consists of only one letter, the major axis of the ellipse may be vertical.
� On motor cycles and their trailers, the dimensions of the axes of the ellipse shall be at least 0.175 m and 0.115 m. On other motor vehicles and their trailers, the dimensions of the axes of the ellipse shall be at least:
(a) 0.24 m and 0.145 m if the distinguishing sign comprises three letters;
(b) 0.175 m and 0.115m if the distinguishing sign comprises less than three letters.
http://www.unece.org/trans/conventn/crt1968e.p df