History of the Oval Sticker or Euro Car Sticker:

It began in 1969 when the UN convened for the purpose of creating a relatively easy way to identify the different sources of traffic in Europe and other places, since there are so many plates used overseas to expect that at the same time, no matter which country you from. With that in mind, they came up with the idea of adding a sticker to your vehicle with a country code, so that officials can identify your origin. The original stickers large ... more than nine inches wide nearly six inches high! Wow, that's really big! If you care to read the official text below from the US's Convention on Road Traffic in 1969, here it is.

1. The distinguishing sign shall be composed of one to three letters in capital Latin characters. The letters shall have a minimum height of 80 mm. (3.1 in.) and their strokes a width of 10 mm. (0.4 in.). The letters shall be painted in black on a white ground of elliptical form with the major axis horizontal.
2. If the distinguishing sign is composed of three letters, the dimensions of the ellipse shall be at least 240 mm. (9.4 in.) in width and 145 mm. (5.7 in.) in height. The dimensions may be reduced to 175 mm. (6.9 in.) in width and 115 mm. (4.5 in.) in height if the sign carries less than three letters.
As regards the distinguishing signs for motorcycles, the dimensions of the ellipse, whether the sign is composed of one, two or three letters, may be reduced to 175 mm. (6.9 in.) in width and 115 mm. (4.5 in.) in height.
3. The distinctive letters for the different States and territories are as follows:
... See here for full list of country codes ...
Any State which has not already done so shall on signature or ratification of, or accession to, this Convention, notify the Secretary-General of the distinctive letters selected by that State.

4. When the distinguishing sign is carried on a special plate this plate shall be fixed in a vertical or nearly vertical position and in a plane perpendicular to the longitudinal axis of the vehicle itself. When the sign is fixed to or painted on the vehicle itself, this shall be done on a vertical or nearly vertical surface at the back of the vehicle.