Master in Sexology and Psychosexual Counseling. For More Info WhatsApp- 9604715783. Though the penis is considered as an organ of masculinity herby many, in reality the function of the penis is like a dropper to carry the sperms from the body of male to the mouth of the uterus of female for fertility. Penis loses its erection when this job is done. The average size of erect penis is about 13cm(5in.) and the girth(circumference) is about. However, a penis of the size of little finger is taken as normal since it equally serves the purpose like any other penis. So, size does not matter.