The largest rodent that ever lived was the size of a cow. It was 3m (9.8 ft) long, with a 1.5 m (5 ft.) tail, and probably weighed around 700 kg (1,543 lbs). This rodent could kill you if it sat on you.
Actually, the figures cited are for Phoberomys pattersoni, which is only the second-largest rodent that ever lived. Its close cousin, Phoberomys insolita, was even a bit larger, but no complete skeleton of it has ever been found, making a precise estimate of its size and weight difficult.
Phoberomys pattersoni lived in the Late Miocene, about 8 million years ago. It dwelled around the Orinoco river delta in modern-day Venezuela, feeding on grasses and shrubs. Its sheer size would have been a deterrent to predators, much like its herbivore contemporaries, the ground sloths.
Like other rodents, Phoberomys pattersoni had large, continuously growing teeth that had to be kept short by gnawing. These teeth, the size of sabers, would have also been used to bite would-be predators. It is not likely that Phoberomys pattersoni had any serious predators, because being so large, it was likely relatively slow, and would have been easy prey for the mega-predators of the time if it couldn't defend itself in a fight. The fact that it even evolved suggests that it could.
Phoberomys pattersoni's natural predators would have included three-meter long crocodiles, saber-tooth cats and huge, flightless carnivorous birds. Living a semi-aquatic lifestyle, it probably ate sea grass.
A full skeleton of the beast was only discovered in 2003. Fragments had been uncovered before, but no one suspected how big the rodent was until the full skeleton was found. Its closest living relative, the South American capybara, can weigh up to 45 kg (99 pounds), 15 times smaller than Phoberomys pattersoni. The capybara is the largest living rodent.
Phoberomys pattersoni managed its huge size by using a different gait than smaller rodents. Its massive stomach would have aided its digestion by functioning as a fermentation barrel to break down stubborn bits of cellulose.