What makes the stories about giants over 12 feet tall very difficult to believe is the physical consequences of increasing height. Animal bodies are physical structures like any other. They function according to physical laws and large changes in scale have major effects on internal physiological functions. This is why insects can get by with 'tubules' under the skin or carapace pumping air into their bodies while vertebrate animals need full-scale lungs and circulatory systems.
And this is one of the problems that paleontologists are always struggling with in trying to guess what dinosaur physiology was like. The tallest land animal alive today, the giraffe, has some pretty amazing (and extreme) adaptations to allow it to stay conscious while holdings its head up at its full height, including a heart just about twice the size of other animals with the same weight. Even so giraffes tend to die of cardiovascular problems, and at much younger ages than other animals of the same size. So how could 'sauropod' dinosaurs have been so large and lifted their heads even higher than giraffes? Some are now saying they simply couldn't have.
Many researchers have questioned how it would be possible for a Brachiosaurus to supply blood to its head. Several unlikely hypotheses have been suggested. Some paleontologists have suggested that Brachiosaurus had a massive heart to produce the needed pressure to lift the blood. Another proposal is that the Brachiosaurus evolved a series of several evenly spaced hearts in the neck as a pumping system that would get the job done. More recently a popular idea is that the Brachiosaurus never lifted its head up but instead just moved it back and forth horizontally.
The sauropod blood pressure paradox has been debated for several years and now it is showing up in physics textbooks. Increasingly, paleontologists are coming to the belief that the Brachiosaurus could not have held its head up, and likewise the other sauropods could not have reared up on their hind legs to reach the higher foliage.
But getting back to humans, there are serious consequences for people who grow above 8 feet or so in height. They tend to have many problems with skeletal stress. Even if their bones are 'healthy' and have normal density they can have fractures from normal falls that average size humans just get up from and go on. And they tend to have serious cardiovascular problems because of the high blood pressure required to stay conscious while standing up. Remember Andre the giant?
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The disease that granted him his immense size eventually began to take its toll on his body. By the late 1980s, André was in constant, near-crippling pain, and his heart struggled to pump blood throughout his massive body.
André died in his sleep due to Congestive Heart Failure on January 27, 1993, in a Paris hotel room. He was in Paris to attend the funeral of his father.
This is quite common with unusually large people even on the scale we have experience of, and the tallest document person was "only" 8 feet 11 inches tall. The idea that a human skeletal frame could be scaled up to 36 feet without structural changes is just ridiculous.
Like ancient Rome, we today are once more importing every form of exotic superstition in the hope of finding the right remedy for our sickness.
-- C. G. Jung
Richard Wilhelm: In Memoriam (1930), CW 15: pg. 60