55-60cm。 One size fits most people. One size fits for knitted clothes, hats and woolen clothes. Because clothes or hats have good elasticity, plump people and skinny people can wear them. When making clothes, M (medium) is the standard, and general clothes are from XS-XXL.
Hat:
A hat is a dress worn on the head, most of which can cover the whole top of the head. It is mainly used to protect the head. Some hats will have prominent edges to cover the sun. The hat can also be used for dressing, hair protection, bald cover, or as part of the uniform or religious clothing. It can be of different kinds, such as high hat, sun hat, etc. Some hats have an overhanging canopy called a visor. Wearing hats has different etiquette in different cultures, which is especially important in Western culture, because wearing hats was a symbol of social status in the past.