This may be a partial bone, but as one can see, it is still as long as one of my student desks. If this bone were complete, I would estimate it to be about 4 feet long. This is the same bone that is in the lower leg of a human, though in humans it is much smaller and thinner than the fossil seen here. There is a considerable amount of wear and damage to the fossil, but overall it is in decent condition.
Diplodocus was one of the larger animals in the Morrison Formation, if not the largest in terms of length. Larger sauropods have since been discovered in other locations (particularly Argentinosaurus of South America), but Diplodocus still remains a well known dinosaur. Diplodocus also allowed for a more accurate reconstruction of sauropod leg posture, due to trackways that have been discovered.
These tracks show that Diplodocus (and other sauropods) carried their legs directly underneath them, much like pillars supporting a building. A sprawling posture, as was previously hypothesized, would not have supported the animal (and probably would have dislocated the hips in the process). Size aside, Diplodocus was a relatively lightly built animal and would have had little difficulty transporting itself around. One thing it probably could not have done was submerge itself in water, as that depth would have put enough pressure on the chest cavity to make breathing difficult.
Diplodocus likely fed itself by using its peg-like front teeth to strip soft leaves from stems. All Diplodocus would have to do is take a mouth full of leaves, and pull its head back. The resulting motion would pull leaves from stems, giving a mouthful of easily digested plant matter. Given the size of Diplodocus, this must have been a fairly efficient method of feeding. The long neck would also have allowed Diplodocus to sweep large areas for vegetation without having to move its whole body much.
In terms of predators, a fully grown Diplodocus would have had little to fear from a single Allosaurus, or any other predator for that matter. There is a strong possibility that Diplodocus could use its whip-like tail as a weapon. An Allosaurus recovered does show a series of injuries consistent with a strike from a sauropod tail, possibly Diplodocus. Juveniles and old/injured individuals would have been more at risk from predators, especially if there was little parental care involved.