What dinosaur was the longest plant-eating dinosaur?

Answer 1

At 60 metres long, Seismosaurus overshadowed every other known plant eater. A very fragmentary giant called Amphicoelias may have been even bigger.

Edit: Actually, Seismosaurus is thought by some to a species of Diplodocus, probably Diplodicus Longus. Anyway, the specimen in question whilst not fully complete, might have been about 30m long.

Supersaurus is a larger and longer animal at about 33-34m long. It has a 12-13m neck!

Amphicoelias was only known from one incomplete vertebra, that has been lost and some question whether it actually existed. Estimates have been between 40-60m! long, depending on how it's reconstructed.

There are also large titanosaurs like Argentinosaurus. Whilst Argentinosaurus isn't very complete, its vertebra are huge suggesting a very large animal. It's impossible right now to know exactly how long it is because no neck or tail bones are known but it might have been around 30m (give or take a few meters) in length.

Even though sauropods like titanosaurs grow larger, the Diplodocids with their incredibly long tails are generally the longest.