There are several ways to measure the tallest dinosaur.
One way is to let the long sauropods, like Diplodocus carnegii (Fig. 1; Marsh 1878; Late Jurassic; 25-32 m long), stand on their hind limbs, like their prosaurod ancestors, balanced by a very long narrow whiplash tail of up to 80 vertebrae. While the neck could not be elevated much beyond horizontal (relative to the dorsal vertebrae), by standing on its hind limbs the torso + neck could be elevated.
“No skull has ever been found that can be confidently said to belong to Diplodocus, though skulls of other diplodocids closely related to Diplodocus are well known.”
The peg-like teeth of Diplodocus
were smaller and fewer than in other sauropods. And the skull was smaller with nares placed higher on the skull. Evidently diplodocids could only handle smaller needles and leaves from conifer trees matching their height. Wikipedia reports, “Unilateral branch stripping is the most likely feeding behavior of Diplodocus.”
We know of junior diplodocids
(Fig. 5), half the skull length but with relatively larger eyes. Cute!
Marsh OC 1878. Principal characters of American Jurassic dinosaurs. Part I. American Journal of Science. 3: 411–416.