What determines the size and shape of hairs?

Hairs are short and stiff in the eyebrows and long and flexible on the head. When the hair shaft is oval, an individual has wavy hair; when it is flat and ribbon-like, the hair is curly or kinky. If the hair shaft is perfectly round, the hair is straight. Humans are born with as many hair follicles as they will ever have, and hairs are among the fastest growing tissues of the body. Each hair follicle on the scalp grows almost 30 feet (9 meters) during an average lifetime.