Is there a hard and fast rule about the size of stamps?

Stamps are different from envelopes. Envelope has a specified standard size, because it needs to occupy space in the process of mail delivery, and stamps are used on the envelope, so as long as the size does not exceed the envelope, it meets the actual requirements of postal work.

As far as the world is concerned, there are also many stamps specially designed, but these stamps are beautifully printed and mainly used for collection.

As far as China is concerned, from the first set of stamps "Dalong Stamp" to the stamps issued by New China in recent decades, they are all designed for postal purposes. So most of them are 30 * 40mm, 40 * 30mm, 30 * 50mm, 50 * 30mm, 40 * 50mm, 50 * 40mm, 40 * 60mm, 60 * 40mm and so on. In addition to the sheetlet, the largest size should be a few stamps issued during the Cultural Revolution, 40 * 60mm, and then. So far, China's stamps are mainly rectangular, but also circular (such as the 2002 World Cup stamps) and triangular (such as the Shenzhou IV stamps), but the stamp area is controlled within a certain range. There is no clear stipulation that stamps must be of a certain size.

Since the stamps themselves are designed and issued independently by the State Post Bureau, there is no need to specify what size they need to meet, as long as it does not affect the actual use.