The first question is how you define "biggest".

Supersaurus vivianae was one of the longest dinosaurs. Scientists estimate it reached up to 34 metres (111 ft) from its nose to the tip of its tail, based on its spinal bones.

Arguably the tallest dinosaur is Sauroposeidon proteles, a massive plant-eater discovered in North America. Thanks to a ludicrously long neck, it stood 17m (55 ft) tall, but relatively few fossils of it have been found. Its estimated height is largely based on the more complete remains of Giraffatitan brancai, which once held the "biggest dinosaur" title.

The heaviest known dinosaur is currently Argentinosaurus, a beast so massive it shook previous dino size tables when it was described in the 1990s. Weight estimates range from 60 to 100 tonnes: again, we cannot be precise because the fossils are so limited.

Now there is another challenger in the dinosaur big leagues. It could be the heaviest, tallest and longest one yet – and that is based on the most complete evidence.

The new contender is a titanosaur, meaning it belongs to the same group as Argentinosaurus.Named for the mythological giants of Ancient Greece, titanosaurs probably lived 70-100 million years ago.

The thigh bones recovered from the site are taller than a man

"These animals are very widespread [and] there are lots of different kinds of them," says palaeontologist Paul Barrett of the Natural History Museum in London, UK. "Not all of them are enormous. Only a very small number reach these very large sizes."

So far the fossil record for titanosaurs is patchy, with a scattering of enormous fossils to hint at their overall size. "We've known about them for a while but they've generally not been a very well-understood group of dinosaurs," says Barrett.

Recently, Patagonia has become a hotspot for these huge dinosaurs. The latest and possibly greatest find yet was made by a farm worker, who stumbled across the tip of a super-sized bone in the desert near La Flecha in 2014.

The thigh bones recovered from the site are taller than a man. Experts from the Museum of Palaeontology Egidio Feruglio, Argentina, say this suggests the creature stood as tall as 5.8m (19 ft) at the shoulder.

It could be the heaviest, tallest and longest one yet

The circumference of the thigh bones, and a forelimb, imply that it weighed around 70 tonnes. It was likely to have been 40m (131 ft) long.

The creature does not yet have a name.

Unfortunately, despite an impressive haul of 223 bones from seven animals, experts have not found a skull. Instead they examined the skulls of other titanosaurs, only three of which have been found so far, to reconstruct the animal.

Preliminary estimates say the newly discovered titan is 10% larger than the former heavyweight champion. But sizing up these prehistoric behemoths is a tricky task.

In the case of Argentinosaurus, all we have is a few fossilised vertebrae from the spine, rib bones, a lower leg bone and some other fragments.

Brachiosaurus when I was growing up was said to be the biggest dinosaur of all time

Some of the bones of Puertasaurus reuili described in 2005 are bigger than those of Argentinosaurus, so it could have been even heavier. Unfortunately, it is only known from a partial spinal column so it is hard to compare.

The most complete titanosaur skeleton currently known to science belongs to Dreadnoughtus schrani, which was first described in 2014 by Kenneth Lacovara of Drexel University, Philadelphia and colleagues. 116 bones have been found, out of an estimated 256.

The original estimates suggested D. schrani could have weighed up to 60 tonnes. But a 2015 study downsized it by 20 to 25 tonnes.

It is not the first time an apparent giant has shrunk.

"Brachiosaurus when I was growing up was said to be the biggest dinosaur of all time," says Barrett. These long-necked sauropods featured in the original Jurassic Park, and were once thought to weigh 80 tonnes. "But that's been shown to be a great over-estimate. Most estimates put it down at 50 tonnes now, as an upper limit."

We're getting very close to the limits of how big an animal can be on land and be viable

Estimates have changed as we have learned more about the structures of bones and their positions within skeletons. Techniques for calculating sizes from partial skeletons have also become more sophisticated.

"Because of the nature of fossils, there's no one gold standard," says Barrett.

The traditional method is based on measuring the circumference of leg bones. "We know from living animals there's a relationship between the circumference of the bones and weight," says Barrett.

The latest technique, known as "convex hulling", relies on computer modelling. Researchers scan each bone to create a virtual model of the dinosaur. If the skeleton is incomplete, the gaps are filled with parts based on related dinosaurs.

The model is then filled with organs, wrapped in muscle and covered in skin, all of which are based on lizards, birds and crocodiles alive today. Experts can then calculate the weight of the dinosaur. This process can fine-tune previous estimates, which is how Dreadnoughtus suffered its drastic weight loss.

Diego Pol is a member of the team responsible for uncovering the secrets of the La Flecha titanosaur. He says they have 70% of the skeleton, which should make their estimates pretty accurate. But until it is formally described in a peer-reviewed journal, the nameless titan cannot knock Argentinosaurus off the top spot.

Preliminary estimates say the newly discovered titan is 10% larger than the former heavyweight champion

"These really big ones don't come along very often," says Barrett, so the title has only changed a handful of times during his career. "We're getting very close to the limits of how big an animal can be on land and be viable."

Tantalisingly, Pol says the titanosaur he is studying was not fully grown when it died.

"By analysing the bone tissue, you can estimate the age of the animal at the time of death," he says. "This is done by performing histological thin sections on limb bones." It is a little like counting the rings in a cross-section of tree trunk.

Pol's findings suggest the anonymous giant may have had some margin to develop. Only an adult fossil will confirm this, so we may see even bigger titanosaurs in the years to come.