Brachiosaurus is an incorrect answer on all counts, it was neither the heaviest nor largest dinosaur. Argentinosaurus is the largest well described dinosaur. Not many of its bones have been found, and length estimates range from 72 feet long to 110 feet long, depending on which other dinosaur was used as a reference point. It is also estimated to weigh the most, between 60 and 88 tonnes, with 73 considered the best guess. The tallest dinosaur is Sauroposeidon, a relative of Brachiosaurus. It was approximately 56 feet tall.