The fishermen once told about a giant sea monster named kraken.

This monster attracts the fishermen from the ship and drowns them in the bottom of the ocean. Now we know that the kraken is real, the tale refers to the species of giant squid.

It is known that it is unlikely that squid killed fishermen, but it is known that they are indeed large, including one of the largest invertebrates known to man.

What we do not know exactly is how much they grow.

The giant squid is still one of the most mysterious large animals on Earth.

The story of an identifiable specimen as Architeuthis dux, known as the greatest squid, dates back to 1639 in Europe, but the giant squid has only been photographed alive in its original environment in 2004.

"Since its discovery, there has been a lot of speculation about the maximum size of the animal," says biologist Dr Chris Paxton of St Andrews University, Scotland.

In an effort to find out how much they can grow, he has researched the example of A. dux animals as part of an analysis published in the Journal of Zoology.

This study includes historical reports and stories of giant squid seen from boats, stranded on the beach, carcasses or body pieces found in the belly of sperm whales, who allegedly hunted and ate these giant animals.

Overall, there are 460 specimens of giant squid that have been measured in one way or another.

The largest squid ever seen is 53 meters long, on the coast of the Maldivian island during World War II on the side of a fishing vessel named Admiralty at night, by a man named J.D Starkey.

Yet many of these measures are not considered accurate and can be ignored.

In the story of such sightings, the size of the squid tends to be exaggerated. For example, according to a book by Darren Naish, J.D Starkey's confession that the squid he saw was longer than the boat he was traveling on and did not include the tentacles, which were usually more than half the length of his body.

Measuring the length of the entire body of this giant squid can also be very difficult, when lifted from the water, the soft tissues of these animals can be elastic and extend not naturally.

So Dr. Paxton ignores unreliable measurements, then he uses various techniques to estimate the maximum size of this giant squid can grow.

This is done by taking into account the overall length of the squid of certain body parts, such as the squid's beak, the hard part of the mouth used to bite and eat the prey, or from its mantle, a large portion of the squid above head and tentacles.

The largest squid measured by convincing techniques has a 2.79m long mantle, Dr Paxton told BBC Earth.

"These are specimens that were stranded in the Cook Strait in New Zealand in May of 1879," he said. "But the overall length is not recorded. From here, we can estimate the length of the squid is 14.28m. "

However this does not mean the squid reaches a maximum length of 14.28m, for two reasons.

First, not all squid have the same proportioned body.

Squid with the same coat size can have tentacles of different lengths, just as people of equal height can have different leg lengths from each other.

On the squid, this difference can be quite large.

Second, it is unlikely that the caught and measured squid is the biggest giant squid ever.

Thus, larger squid is very likely still in the open sea.

And according to Dr. Paxton's analysts, they can indeed be very large.

"Given the differences in each of the squid in their population, the squid with a coat length of 2.79 meters in total can range between 5.83 meters to 27.53 meters," he said.

This signifies a giant squid can grow over 20 meters in length and "very reasonable" some of which can reach 27 meters or more.

"I would emphasize, as a responsible scientist, this is all based on the count," says Dr. Paxton. "And as I said to my students, the count can also be doubted".

But Dr Paxton says his counting is quite safe, even when other squid experts disagree, hoping it can be tested and confirmed by new discoveries in the future.

This study also leads to one further conclusion.

Underwater battle

Myths and fairy tales about giant squid also include their fights with sperm whales, in the battles between the underwater giants.

Giant squid can still be found in the abdomen of sperm whales, it is clear that sperm whales hunt and eat species A dux, as well as hunt and eat species that are also large, but smaller as Mesonychoteuthis hamiltoni.

But some types of squid grow too big to be caught by sperm whales.

Dr. Paxton's analysis shows that the largest squid may be too large for sperm whales that are about 11 to 12 meters in length.

Only the largest sperm whales, which can grow up to 20 meters long, can hunt down any kind of giant squid