Basically, I'm trying to see if a land-based creature roughly between 50-100 ft. long could exist on Earth at the moment and still be scientifically plausible. I know that whales and other creatures of their size are able to reach the sizes they do because of the ocean; and creatures that reached similar sizes in prehistory had increased oxygen levels on their side, as well as large bodies of water to wade in if they ever felt the needs (namely, sauropods, the biggest animals to ever live). They also had more readily-available food to accommodate their huge appetites and crazy metabolism, and didn't have to worry about things like human settlement or deforestation. Having said all of that, is it still possible?
EDIT: To clarify, a multicellular creature with complex biological systems shared across most animal species. I'm not talking about plant-life or fungus, or anything like that. Dinosaurs, maybe...or basically any giant version of a pre-existing animal, if that helps.