system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What's the biggest/tallest a humanoid creature(giant) could get on Earth and why?

I've heard that giants the height of mountains are impossible because of something to do with blood circulation? Just wondering how tall they could actually get.
gpt
It depends a lot on how much leeway you allow for the whole humanoid part and how long you want them to last.

The human body-plan is pretty much optimized for the range of sizes that humans actually get.

There are a couple of medical conditions that can induce people to grow taller than normal, but those usually come with an early deaths.

Even normal very tall people tend to have all sorts of risk factors and health issues and lower life expectancy than shorter ones.

Robert Wadlow is the tallest confirmed human at 2.72m but it basically killed him at age 22.

In the past there existed a species of primate called Gigantopithecus which may have grown to up to 3 meters tall.

Neither Wadlow nor Gigantopithecus really had the same body plan as a normal human. Wadlow was incredibly think and strectched looking and Gigantopithecus would have likely looked more like a scaled up orangutan than a human (we did not find enough bones to be really sure), but both still count as humanoid.

Scaling it up beyond that is difficult because of the cube-square law.

If you take an object (like a human) and scale it up so that it is twice as tall it will have increased in volume by factor of 8 and in surface area by a factor of 4. Since lots of things that make a body plan work depend on the ration of for example skin-surface to total mass being right, you end up with problems. If you make things too small or too big the body will either loose too much heat or too little.

Objects will risk breaking under their own weight if you scale them up because the mass increases with the cube while the ability to carry it only with the square of the scale up factor.

There are lots of other factors that end up not working if you scale it up too much.

Think of the way a toddler can easily deal with running stumbling over their own feet and falling flat on their face. They will cry but mostly come away from the experience with no big harm done. An adult making a similar fall flat on their face from standing up straight will be at far greater risk of injuring themselves.

Scale a human up to the size of a building and falling on their face will basically be a death sentence. 

This keeps us from having to worry about giant ants or giant spiders but also keeps us from having giant humans.

Anything much taller than the tallest NBA players is basically not healthy and you would need to redesign the human body to get much taller than 3 m.