Shape and Size

Postage stamps of various shapes and sizes, from Italy, Yemen Arab Republic, France, and Hungary.
The usual shape of a postage stamp is a rectangle, this being an efficient way to pack stamps on a sheet. A rectangle wider than tall is called a "horizontal design", while taller than wide is a "vertical design".
A number of additional shapes have been used, including triangles, rhombuses, octagons, circles, and various freeform shapes including heart shapes, and even a banana shaped stamp issued by Tonga from 1969 to 1985.
The usual size ranges from 10-30 mm in each direction, experience having shown this to be the easiest to handle. Many countries use only a limited selection of dimensions, to simplify automated machinery that handles stamps.
The smallest postage stamp on record was issued by Mecklenburg-Schwerin in 1856, and was a square, with sides measuring 10 millimeters.
The biggest stamps in history were used in the USA from 1865 and measured 52 by 95 millimeters, but were used exclusively for mailing newspapers.