In a humanoid mammal, say 15-20 feet tall, how would the brain, heart, reproductive organs, facial feature and other attributes scale up? In very large mammals, the brain seems to be comparatively small while the heart is very large. Would this hold true for a human-like animal?
Scaling a humanoid up to 24 feet is very possible, even if evolutionarily unlikely. The main issues that have to be dealt with are muscle strength and the cardiovascular system. Most of the other functions either scale correctly with size, or can afford to take a small hit in efficiency.
Bones: every time height doubles, the bones have to take twice as much weight proportionately. Luckily, your bones are are already very strong, so increasing their load by a factor of four wouldn’t lead to broken bones(most bones break from excessive shear stress or collision with something. As long as the humanoid doesn’t make a habit of kicking rocks or jumping from heights(which wouldn’t be as high for them) it would be fine.
Stomachs: food would have to be pushed through the stomach four times faster. This is definitely doable without changing much of the stomach or increasing mass.
Brain: doesn’t have to increase in size at all! This saves some weight and extra energy usage at the expense of an oddly small looking head. We may keep the head proportional and use that extra space to help out the cardiovascular system.
Cardiovascular system: humans aren’t very good at surviving vertical G-forces, but we can survive G’s 2-3 times normal. If trained to survive g’s, people can go higher than that, but not by much. Unfortunately, the humanoid is 4 times taller than a normal human, so are standard heart may not cut it. This would be solved by having two hearts, one slightly lower than a humans, and one inside the head(headspace has increased 64-fold, so there’s plenty of room). Two hearts means two lungs. The head-lungs could be connected to the mouth, while the chest-lungs may have spiracal-like openings. The cross sectional area of veins only increases by a factor of 4 while mass increases 8-fold. That means that blood is only able to pass through are veins at 1/4 of the proportional speed. We can either increase their proportional size, increase the speed at which blood flows(higher blood pressure, higher heart rate) or increase the red-blood fell count. A combination of all three would probably be best.
Muscles: These are arguably the most difficult to scale-up. I’m reasonably sure(not certain) that muscle strength depends on cross-sectional area, which scales 4-fold every time height doubles. That means that our muscles are lagging behind at 1/4th their proportional strength. The easy solution is to say that this giants muscles are 4-times stronger per pound than human muscles. Otherwise, you have to scale the muscles up, which increases weight, requiring more scaling up. An in-shape human can carry someone equal to their weight on their shoulders, though it’s not a very pleasant experience. I’m sure strength-based athletes can carry someone their weight much easier than I can. So your humanoids muscles really only need to increase in strength 2-fold. The legs are the most important, but also some of the heaviest part of your body. To be generous, id say the cross-section of the leg muscles would have to increase 4-fold.
Features: eyes would be very small proportionately, nose may be a similar size, if not larger to make breathing easier. Mouth would probably be around the same proportional size. Ears would be smaller proportionately. Reproductive organs would probably scale near-proportionately.
And boom! A humanoid that walks on two legs all the time and stands 24 feet tall, can’t run that fast, and weighs over 11,500 pounds(180*64—plus whatever the scaling up or certain parts adds).
most likely, it'll be essentially an elephant-looking ape, but with no trunk and maybe a longer neck
You see, the main problem here is that, when compared to dinosaurs, their hollow but strong bones and airsack systems, mammals tend to have bigger problems growing. Yes, African elephants can get bigger than what you asked, and that comes at the price of being quadrupedal. One thing that you'll notice is that, unlike dinosaurs, mammals have a harder time growing due to having heavier bones and lacking an airsack system, which, while great for flight, is also very effective at helping you keep cool.
Looking at the brain, circulatory system and other systems overall, they'll likely be very similar in size and function to an elephant's or maybe an Paraceratherium's. Given we're talking mammals, I find it highly unlikely that the humanoid will be bipedal, because every mammalian instance of a creature that size was quadrupedal (4 legs hold weight better than 2) and because humans and primates usually can't deal too well with bigger size (the biggest one ever recorded was the Gigantopithecus blacki which was 10 feet high). As a result of the quadrupedal anatomy, the skull and brain will be sustained mostly by muscle, meaning its brain can't have the same weight ratio than in humans, but since it's elephant sized, it'll likely be the size of an elephant.
Regarding facial features, I'm betting on more dexterous lips and tongue, likely with a prehensile tongue similar to a giraffe. It's digestive system will likely develop to digest plant matter (again all instances of mammals this large were herbivores). It's overall facial features will likely resemble something like the gigantopithecus, except the neck will likely be moved a bit. It's eyes though will likely migrate more to the sides, reducing the range of binocular vision but increasing the overall field of vision.
So summing up, your humanoid will likely be a humanoid herbivorous ape, fully quadrupedal, probably still capable of standing on two legs for short periods (elephants can do it so I don't see why your ape couldn't), with heavily padded hands and feet to help it walk around (I don't think it'll be a knuckle walker though, it'll likely adapt to use the entire hand to sustain itself). I'm mostly sure it'll adapt over time to use more it's mouth, with prehensile lips and tongue so that they don't rely on forelimbs to grab food. It's overall systems will be very reminiscent to an elephant's and it's facial features likely won't change that much, though there's a good chance its eyes will move more to the sides of the face to allow for a wider field of vision. There's also a chance it'll either develop a longer and stronger muscular neck (notice this might cause the brain to become smaller) or tusks for self defense.