Size and Weight:
One of the odd things about paleontology is that incorrect names can sometimes take precedence over correct ones. For example, when its bones were first discovered in 1843, the prehistoric whale Basilosaurus was incorrectly identified as a marine reptile--hence its name, Greek for "king lizard." Scientists then tried to amend their mistake by calling it Zeuglodon (after a feature of this enormous mammal's teeth), but usage has since reverted to the earlier, incorrect moniker.
Whatever you choose to call it, the 50-ton Basilosaurus was one of the largest animals of the Eocene epoch, rivaling earlier, landbound sauropod dinosaurs like Seismosaurus and Argentinosaurus in size. Because it had such small flippers, relative to its bulk, it's believed that Basilosaurus swam by undulating its long, snake-like body; today, it's the official state fossil of both Mississippi and Alabama, where various specimens have been discovered.
Sauropods are one thing; how did Basilosaurus measure up against the giant marine animals of prehistoric times? Well, it was certainly bigger than either Kronosaurus or Liopleurodon (the biggest members of the breed of marine reptiles known as pliosaurs), and it seemed to be in roughly the same weight class as the monster shark Megalodon, which lived at least 10 million years later. Sadly, these two giants never got a chance to meet, so we'll never know which would have prevailed in a face-to-face battle.