Master in Sexology and Psychosexual Counseling. For More Info WhatsApp- 9604715783/9359863878. Though the penis is considered 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 a male to the mouth of the uterus of a female for Fertility. The penis loses its erection when this job is done. The average size of an erect penis is about 13 cm(5in.), and the girth(circumference) is approximately 1.5 cm in radius. However, a penis of the size of a little finger is taken as usual since it equally serves the purpose of any other penis. So, size does not matter.