Well the biggest animal that has ever existed on earth is the blue whale, which still exists today. If you were somehow transported into the aorta of a blue whale, you could swim around in it…for a little while, until you got tired and then you would be washed down the aorta and in the second or third branch, your body would get stuck, which would have roughly the same affect as a massive blood clot and would kill the whale.
What I’m trying to say is, while you /could/ swim around inside of the circulatory system of a blue whale, don’t.
I just realized I didn’t answer your question. The largest dinosaur was a titanosaur, but we’re not sure which one because we only have fragmentary fossils of several species. Apatosauruses weren’t titanosaurs, but they were closely related. The largest titanosaurs probably peaked at around 100 tons. An adult blue whale can weigh over 200 tons.
Also, the biggest living thing on earth is a fungus. Though the mushrooms we see are usually teeny tiny, these are just the reproductive structures of the fungus. It would be like comparing the size of cherries on a tree to the size of the cherry tree itself.
The ORGANISM is all below ground, in what is called a mycorrhizal mat. One of the first ones we found was in the Upper Peninsula of Michigan, and one of the towns there still holds the Humungus Fungus Festival each year. The current largest is in Oregon and covers
2,384 acres (965 hectares).
A decent article can be found here.
The Archery Range 