Please note this is not the legal minimum size as the Vienna Convention mandates a huge sticker for 3 letter country codes. We have used the smaller 2 letter size (part b) but this does not meet the legal requirement for three letters. In reality a police stop is not going to know the law in this detail. I have never been challenged on my small version in 4 years of travelling.
This sticker is thus fully legal for Motor Cycles but technically not legal for Cars.
Vienna Convention 1968
4. 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.