Giant squid could measure up to 20 metres in length.
Fewer than 500 of the mysterious invertebrates (genus Architeuthis) have ever been measured. To calculate their maximum possible size, Charles Paxton of the University of St Andrews, UK, compiled recorded measurements of giant squid, and established relationships between measurements such as the length of the whole body, the mantle and the beak. These relationships, and the variation between observed squid lengths and beak sizes, suggest that the animals could plausibly be 20 metres long.
Paxton says that some giant squid may grow too large to be eaten by some of their predators, such as female sperm whales.
J. Zool. http://doi.org/bhwq (2016)