system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What limits creature size here on Earth?

Why is it that we seem to have stopped at the blue whale? Is it possible that larger creatures could exist? How much larger? Would they have to be sea creatures? 

Also, If we stretched the rules a little bit, what is the largest a creature could be here on earth? (Land or sea).
gpt
Oxygen saturation and transfer efficiency makes it hard to have a large creature that can move quickly.  Thermal transfer and dumping said heat puts a limit on size for creatures. Gravity and the load limit on things like bones sets an upper end on how big a thing can be without crushing. Nutrition requirements mean things eat more as they get bigger but it gets harder to find the food when you are huge.  Blue whales exist at an outer edge of the envelope given present conditions and what biology is able to come up with by chance, relying on water to hold the weight and pull away heat, floating to limit how much energy they put out, and eating craptons of little plankton that can be sucked through in a constant buffet.