Amphicoelias, possibly the largest sauropod and largest animal yet discovered, shown with two of the largest mammals alive today. The Blue Whale at a massive 28 meters long and weighing ~140 metric tons, and the Asian elephant at 3m tall and ~4 tons in mass.
Edit: I redid this scale with a more modest reconstruction of Amphicoelias. Still seemingly the largest animal ever at about 65-70 meters long. Only known from one dorsal vertebra estimated to be nearly 9 feet tall, scaling from related sauropods gives numbers clustering in the 60-70 meter range. At the size I depict here a likely weight would be around 150-180 metric tons(Assuming slightly higher pneumaticity than seen in Diplodocus). Recent studies show that sauropod necks tend to lengthen to the power of 1.35 in comparison to smaller relatives. Since diplodocus had a ~6.5-7 meter neck, Amphicoelias could have had a neck 2.5^1.35 times as long....That's over 20 meters of neck! The only possible contenders to Amphicoelias's size are the Blue Whale and the very poorly known Broome Titanosaur.
All scaling is imprecise and based on Amphicoelias assuming similar proportions to Barosaurus and Diplodocus.