The problem isn't with the size but, for lack of a better word, the design of the animal. In theory a creature could be as large as it's design and space permit it to be so a well designed creature in a large area with plenty of food could reach the size of a city (probably not on earth though) for earth the largest is probably the size of a whale or large dinosaur. Sorry for not being able to provide a clear answer but there really is no clear answer to this question.

The size of the biggest dinosaurs. Also, iguanas (which Godzilla is mutated from), are cold blooded, not warm blooded, and therefore would not have that amount of body heat.

Depends on the creature.

I'm NOT asking what is the largest animal eer discovered or currently in existance, I want to know, according to biology and physics, how big CAN an animal be in theory.

I ask because I recently heard that Godzilla could not exist because A, It's bones would have to be ade of carbon nao-tubes to support its own weith, B) because it's body heat would be so great that it would melt metal.

So, how big could a creature be.