Size and Weight:
Paleontologists mostly refer to Seismosaurus, the "earthquake lizard," as a "deprecated genus"--that is, a dinosaur that was once thought to be unique, but has since been shown to belong to an already existing genus. Once considered among the biggest of all dinosaurs, most experts now agree that the house-sized Seismosaurus was probably a species of the much better-known Diplodocus.
Not to further disillusion you, but it's also a distinct possibility that Seismosaurus wasn't quite as big as scientists once assumed. Some researchers think this late Jurassic sauropod weighed as little as 25 tons and was considerably shorter than its stated length of up to 120 feet, though not everyone agrees with these drastically scaled-down estimates. By this accounting, Seismosaurus would have been a mere runt compared to the gigantic titanosaurs that lived millions of years later, such as Argentinosaurus and Bruhathkayosaurus.
The extreme length of Seismosaurus' neck--at 30 to 40 feet, it was much longer than the necks of most other sauropod species, with the possible exception of the Asian Mamenchisaurus--raises an interesting question: could this dinosaur's heart possibly have been strong enough to pump blood all the way to the top of its head? If not, it's likely that Seismosaurus held its neck roughly parallel to the ground, sweeping its head back and forth like a vacuum cleaner, rather than in the more taxing vertical position.