There are no standard dimensions for a postage stamp. Stamps are available in many different shapes and sizes. There are even round and triangular stamps.
Stamp sizes have varied greatly since the United Kingdom first issued the Penny Black in 1840. The most common stamps used in the U.S. often average 22 millimeters by 25 millimeters in size, and they have a vertical or horizontal design. Commemorative stamps come in a wide variety of shapes, as their dimensions vary depending on the design chosen. Because stamps are available in so many different dimensions and styles, a wide variety of mounts are available for stamp collectors to display their collections.