When it comes to penis size the blue whales penis is the largest of any known living animal. In order to provide the whale with a more streamlined design when. In another specimen in which the length of the whale was 20 metres (66 ft) and weighed metric tons, the brain weighed only kilograms ( lb). "Blue whales have the largest penises on Earth. An erect blue whale penis is 12 inches (30 cm) in diameter and ten feet in length.