The largest living thing on earth is the blue whale. Coming in at an average weight of 220,000 pounds, the king of the deep is a truly monstrous creature. And, in fact, science can all but definitively prove that the blue whale is probably the upper size limit of what carbon-based, ATP-powered biological life can reasonably reach.
Because of the limitations of a biological entity powered by ATP and metabolism as we understand it, this means that movie monsters like kaiju and giant apes are extremely unlikely to ever be a reality. As for why this is the case, let’s take a look at the science.
The first limiting factor in the size of any living thing is its metabolism. Life as we know is powered primarily by a chemical chain called ATP, or adenosine triphosphate. This molecule is broken down in the muscles into two constituent parts, leaving behind ADP, adenosine diphosphate. In this process, the energy required to make muscles move is released. This might sound simple, but it’s the fundamental engine that powers literally all of your movement.
Without ATP, you wouldn’t be able to move. Birds couldn’t fly, fish couldn’t swim, and life simply wouldn’t be. And, indeed, the larger the life form, the more ATP it has at its command. A caterpillar is much slower than a mouse, who is, in turn, much slower than a cat. A cat is notably slower than a dog, while a cheetah is the fastest land animal by far.
However, there is an upper limit to this that suggests size isn’t the sole determination for speed: animals much bigger than cheetahs begin to slow down. Take, for instance, the African elephant, one of the largest animals on Earth. It isn’t faster than a cheetah; in fact, many animals can outrun an elephant. This suggests that, at certain size thresholds, the extra musculature afforded by increased size begins to weigh too much to see increases in speed, making life forms larger than elephants or whales unlikely to be successful.
To get the ATP they need to move around, animals have to eat. And the amount they have to eat is directly correlated to how big they are. A mouse eats much less than a human, who eats much less than a whale. A theoretical giant lizard monster would need to intake so much food that their existence would be primarily centered around obliterating ecosystems just to break even on the energy it would need to maintain homeostasis.
Another issue that hypothetical giant monsters would run into is the shape of their skeleton being unable to support their mass. Because of the shape of bones, and their structure, they are very light for how durable they are. As such, when scaled up to a creature that would stand at 50 feet tall, all skeletons observed in living things on Earth would simply shatter under the weight.
Other problems, like the immense blood pressure such a creature would possess, as well as the weight of their own organs likely crushing each other, make the concept of such a giant creature simply unrealistic. While life forms of such towering proportions may be possible under a different framework, life as we know it simply can’t support any living animal that big.