Size and Weight:
Although paleontologists agree that Liopleurodon must have been a fearsome predator of the late Jurassic seas, there's less consensus about how big this pliosaur actually was: estimates range anywhere from 20 to 35 feet long, with proportional variations in weight. Matters haven't been helped by an episode of Walking with Dinosaurs that featured a Liopleurodon nearly 75 feet long, which experts dismiss as complete nonsense (compounding the fiction, the same Liopleurodon was depicted as leaping out of the water and swallowing a passing Eustreptospondylus whole!)
Whatever its exact measurements, it's clear that Liopleurodon was built for predation. Despite its bulk, this marine reptile was likely able to propel itself quickly and smoothly through the water with its four powerful flippers, holding its mouth open to catch any unfortunate fish and squids that happened across its path. (Not everyone agrees that Liopleurodon swam this way, with its mouth open, but proponents point out that this would have allowed it to better smell its prey, and in any event Liopleurodon would have had to shovel hundreds of pounds of food a day into its maw to keep up its 25-ton weight.)
By the way, the late Jurassic contemporaries of Liopleurodon--the sauropods--were the biggest land animals that ever lived, still unmatched in size after the dinosaurs went extinct 65 million years ago. Oddly enough, though, other, more recent marine creatures have surpassed the bulk of Liopleurodon, most notably the giant shark Megalodon, some specimens of which may have attained weights of over 50 tons, and the equally huge prehistoric whale Leviathan.