It is my understanding that one of the reasons for why massive lifeforms (as depicted in fiction) aren't possible is the fact that the heat radiated by their body after a certain point would be too much for it to be survivable. Basically cooking them from the inside.

What is the maximum theoretical size a creature can realistcally be?

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    $\begingroup$ To deal with heat, water has a great specific heat capacity, meaning it can hold a lot of energy—that is why we cook with it. I'd give your animal a second circulatory system that pumps water—or some mixture that may hold more energy—throughout the animal's body. Then, I suppose you'd have an animal that would constantly sweat and would constantly require water. $\endgroup$ – B.fox Dec 30 '18 at 14:20
  • $\begingroup$ @B.fox ^ so, that would be the circulatory system then? :) $\endgroup$ – Pelinore Dec 30 '18 at 14:23
  • $\begingroup$ Also, these may be of interest to your question worldbuilding.stackexchange.com/q/316/44399 worldbuilding.stackexchange.com/q/317/44399 worldbuilding.stackexchange.com/q/1360/44399 worldbuilding.stackexchange.com/q/109716/44399 worldbuilding.stackexchange.com/q/120492/44399 should one or more of them not be a duplicate. $\endgroup$ – B.fox Dec 30 '18 at 14:24
  • $\begingroup$ @B.fox Interesting, thanks for the links and the idea. $\endgroup$ – Azumentris Dec 30 '18 at 14:26
  • $\begingroup$ "thanks for the links and the idea" @Azumentris : I think you'll find the idea was fox polity pulling your leg, what he's described the circulatory system as possessed by all humans, mammals, lizards, etc on earth :) $\endgroup$ – Pelinore Dec 30 '18 at 14:29