How long is a baby diplodocus dinosaur?

We don't really know. However, it's likely that newborn dinosaurs of giant parents were comparatively small. There are two reasons, the more important being that dinosaurs all seem to be born from eggs, and there are absolute limits to the size of an egg. If an egg is too big, then the shell must be thick or else it would be too fragile and if the shell is too thick, the newborn can't break out of it. The largest egg ever found is that of a bird, and is about twice the size (eight times the volume) of an ostrich egg. This is theorized to be the maximum possible egg. The other reason that young dinosaurs like diplodocus would be small is that their tails and necks would be much shorter, proportionately, than those of their parents, so more of their body mass would be in their torso. This is called neoteny, and is apparent in almost all higher animals. At a guess, a newborn diplodocus would not exceed two feet long.