Premium High Tack Laminated Vinyl not the cheap stuff you get from the AA and Ebay/Amazon.
This is the legal minimum size as the Vienna Convention sticker.
This sticker legal for Motor Cycles and 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.
“””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 in black on a white background having the shape of an ellipse with the major axis horizontal”””