The blue whale can have a penis as large as 10 feet. The biggest penis belongs to the biggest animal that has ever lived on the planet, the blue whale. The average length of the penis of a blue whale is 8 feet and can go up to 10 feet. However, the diameter of the organ is between 12 to 14 inches only.  After the blue whale, the biggest penis belongs to animals such as Lake Duck, Barnacles, Greater Hooked Squid, African Elephant, and Banana Slugs.