Argentinosaurus was a gigantic long-necked, long-tailed dinosaur that lived in what's now Argentina roughly 95 to 90 million years ago, during the Late Cretaceous.
Big, but not fully known
Argentinosaurus is only known from a handful of fossilized bones — mostly vertebrae, part of the shin, and some ribs — no complete skeleton has ever been found. That makes its exact size genuinely uncertain, but based on those bones, researchers commonly estimate it at roughly 30 meters (about 100 ft) long and possibly over 70 tonnes, which would make it one of the largest land animals ever known to have existed. Some individual vertebrae are large enough that a person could comfortably stand inside the hollow spaces within them.
How something got that big
Argentinosaurus belonged to a group called titanosaurs, plant-eating dinosaurs with long necks and tails that included some of the largest animals to ever walk on land. Their size is thought to have been possible partly because of an efficient respiratory system, similar in principle to a bird's, that let them breathe more efficiently than mammals of comparable size.
Why the uncertainty matters
Because "largest dinosaur ever" claims are usually based on partial skeletons like this one, size estimates for animals such as Argentinosaurus tend to shift somewhat as new fossils are found or as scientists refine how they scale up from incomplete remains — it's less about scientists disagreeing and more about the fossil record itself being incomplete.