Actually, the larger the animals, the easier it is for it to have a brain with human level intelligence.
There is a minimum size of brain necessary for human level intelligence.
Nobody knows what that minimum size is.
There are approximately 6,500 known species of mammals at the present.
The vast majority of them have tiny bodies with less mass than a human brain. The mass of an average human brain is about
The adult human brain weighs on average about 1.5 kg (3.3 lb).
And many species of mammals have brains much smaller than that since their entire bodies are not that massive.
But there are hundreds of large mammal species ranging in size from small humans to larger than humans. Mammals species with such large bodies can possibly support large brains. and some mammal species with such large bodies actually do support large brains.
And there are about ninety or a hundred species of mammals, primates, proboscideans, and cetaceans, with brains roughly in the range of human size, and in some cases far larger.
So it is possible than between one and about a hundred species of mammals on this planet are approximately as intelligent as humans and thus count as persons.
Some person claim that the absolute size of the brain doesn't matter as much as the ratio of brain to body size. If an animal has a larger body it may need a larger brain to senses sensations from its larger body and to control its larger body. Thus it is possible that even the largest brained nonhuman mammal does not have a brain sufficiently large for human level intelligence.
Some people might think that the size of brain necessary to control the body would scale with the dimensions of the animal. An animal twice as large would need twice as large a brain to control its body, in addition to the parts of the brain that give intelligence.
Some people might think that the size of brain necessary to control the body would scale with the square of the dimensions of the animal, and thus with the surface area of the body. An animal twice as large would need four times as large a brain to control its body, in addition to the parts of the brain that give intelligence.
Some people might think that the size of brain necessary to control the body would scale with the cube of the dimensions of the animal, and thus with the volume and mass of the body. An animal twice as large would need eight as large a brain to control its body, in addition to the parts of the brain that give intelligence.
Note that having the brain scale linearly requires much less brain than having it scale as the square of dimensions, and having the brain scale with the square of the dimensions requires much less brain than having the brain scale with the cube of the dimensions.
I also note that some body plans are much more complex than others and might need much larger brains to control than others.
Brain-to-body mass ratio, also known as the brain-to-body weight ratio, is the ratio of brain mass to body mass, which is hypothesized to be a rough estimate of the intelligence of an animal, although fairly inaccurate in many cases. A more complex measurement, encephalization quotient, takes into account allometric effects of widely divergent body sizes across several taxa.1 The raw brain-to-body mass ratio is however simpler to come by, and is still a useful tool for comparing encephalization within species or between fairly closely related species.
Encephalization quotient (EQ), encephalization level (EL) or just encephalization is a relative brain size measure that is defined as the ratio between observed to predicted brain mass for an animal of a given size, based on nonlinear regression on a range of reference species. It has been used as a proxy for intelligence and thus as a possible way of comparing the intelligences of different species. For this purpose it is a more refined measurement than the raw brain-to-body mass ratio, as it takes into account allometric effects. Expressed as a formula, the relationship has been developed for mammals and may not yield relevant results when applied outside this group.
Encephalization quotient was developed in an attempt to provide a way of correlating an animal's physical characteristics with perceived intelligence. It improved on the previous attempt, brain-to-body mass ratio, so it has persisted. Subsequent work, notably Roth,1 found EQ to be flawed and suggested brain size was a better predictor, but that has problems as well.[unbalanced opinion?]
Currently the best predictor for intelligence across all animals is forebrain neuron count. This was not seen earlier because neuron counts were previously inaccurate for most animals. For example, human brain neuron count was given as 100 billion for decades before Herculano-Houzel found a more reliable method of counting brain cells.
It could have been anticipated that EQ might be superseded because of both the number of exceptions and the growing complexity of the formulae it used. (See the rest of this article.)[unbalanced opinion?] The simplicity of counting neurons has replaced it. The concept in EQ of comparing the brain capacity exceeding that required for body sense and motor activity may yet live on to provide an even better prediction of intelligence, but that work has not been done yet.[unbalanced opinion?]
Clearly there is still a lot of research to be done to predict how much brain is needed to control a body of specified size, and how much more brain is needed for intelligence.
It seems to me that if a humanoid giant has a body that is ten times as massive as a human body, it might possibly need a brain tens times as massive as a human brain to control its body, and if has a body 100 times as massive as a human body it might possibly need a brain 100 times as massive as a human brain to control its body. But if the extra part of the brain needed for intelligence didn't have to be any larger than the extra part of the brain needed for intelligence in a human sized body, the giants could have total brain mass and energy consumption that was proportionally less than that of a human.
And it is possible that the brain mass needed to control a body actually increases a bit less than with the cube of the body's dimensions, which would mean that your giant's brain size could be a bit smaller compared to its body mass and it could still be intelligent.
So unless some expert in brain size relative to intelligence says that the best current estimates and calculations show a specific upper body size for an intelligent being, I would assume that a giant humanoid could be just as intelligent as a human with a somewhat smaller relative brain size.