It is certainly possible for intelligent beings to be only six inches tall. Of course that depends on how many feet long they are.
It wasn't specified that the aliens have humanoid shape, so I imagine that they look like centipedes:
Arthropleura (Greek for jointed ribs) is a genus of extinct millipede arthropods that lived in what is now northeastern North America and Scotland around 345 to 295 million years ago, from the Viséan stage of the lower Carboniferous Period to the Asselian stage of the lower Permian Period.2 The larger species of the genus are the largest known land invertebrates of all time, and would have had few, if any, predators.
Arthropleura ranged in length from only about 0.3 metres long to A. armata which was 2.5 metres long.3 Arthropleura was able to grow larger than modern arthropods, partly because of the greater partial pressure of oxygen in Earth's atmosphere at that time and because of the lack of large terrestrial vertebrate predators.4
The flattened body of Arthropleura is composed of approximately 30 jointed segments, each of which was covered by two side plates and one center plate. The ratio of pairs of legs to body segments was approximately 8:6, similar to some present-day millipedes.5
A maximum length of 2.5 meters is about 8.201 feet. So Arthropleura probably grew massive enough to support brains large enough for intelligence.
Arthropleura were limited in size by relying on passive diffusion of gas for oxygen and by having exoskeletons. A vertebrate with lungs that had the same body plan could grow much larger than Arthropleura. So intelligent beings six inches tall and several feet long and wide could evolve and exist on Earth.
And on a planet with high enough surface gravity, such a body form might possibly be the only form possible for a species with enough body mass to have a brain large enough for intelligence.
In Hal Clement's novel Mission of Gravity the natives of the planet Mesklin, which has a surface gravity varying between three and seven hundred times the surface gravity of Earth, are shaped like centipedes.
I note that there are seven snake species that sometimes reach weights of over 100 pounds (45.3 kilomgrams) and lengths of at least 15 feet 94.572 meters). Such snakes have sufficient body mass to support brains large enough for intelligence. So giant snakes with arms and hands could be large enough for intelligence and possibly not more than six inches high.
I note that a snake like being six inches in diameter and 100 feet long would have volume of 25 cubic feet, which is close to ten times the volume of an average human which is close to 2.5 cubic feet. A cylindrical being 10 feet long and 0.5 feet or six inches in diameter would have a volume as large as the average human and thus could support a brain as large as a human brain.
The marine bootlace worm Lineaus longisseumus is very thin, but sometimes grows very long.
Bootlace worms may grow very long but are usually only 5 to 10 millimetres (0.20 to 0.39 in) in width.
In 1864 a specimen washed ashore in the aftermath of a severe storm by St Andrews, Scotland, which was more than 55 m (180 ft) long,6 longer than the longest known Lion's mane jellyfish, the animal which is often considered to be the longest in the world. However, records of extreme length should be taken with caution, because the bodies of nemerteans are flexible and can easily stretch to much more than their usual length.
A worm 10 millimeters in diameter would have a cross sectional area 78.53975 square millimeters. If it was 55 meters or 55,000 millimeters long it would have a volume of 4,319,686.2 cubic millimters, or 0.0043 cubic meters, which would be about 0.1525 cubic feet, a small percentage of the volume of a human.
If the aliens are six inch tall centaurs, they will have much more body mass than six inch tall humanoids, and thus be more likely to have brains large enough for intelligence.
Six inch tall humanoids would have about one eleventh or 0.090909 of the height, and thus about 0.00075 the volume, of average humans. That is one divided by 1,333.3333 of the volume and mass of an average human.
I suppose that the brains of six inch tall humanoids might be spread throughout their bodies and not concentrated solely in their heads, and thus could be significantly larger proportional to their stature. After all, a human brain functions well and has dimensions equal to the dimensions of a six inch tall body.
On Earth the vast majority of mammals have bodies much smaller than humans. But there are hundreds of mammal species about as large as humans or larger, and thus large enough to have brains about human size or large. A minority of those large mammal species, roughly a hundred, do have brains about human size or longer. And many of those species do exhibit intelligent behavior similar in many respects to that of humans, so that the popular idea that there is only one species of intelligent beings or people on Earth is not necessarily correct. There might possibly be between one and a hundred other species of mammals on Earth that should be classified as intelligent beings.
Animals that are not mammals and whose brains are organized differently than mammal brains, might possibly need larger brains ins some cases to have human intelligence, and smaller brains in other cases to have human intelligence, depending on how well their brain organizations optimize intelligence per brain mass. Thus it is even more uncertain with non mammal species what the minimum brain size for intelligence is.
Some species of birds show surprisingly high intelligence for their sizes.
The corvids (ravens, crows, jays, magpies, etc.) and psittacines (parrots, macaws, and cockatoos) are often considered the most intelligent birds, and among the most intelligent animals in general; pigeons, finches, domestic fowl, and birds of prey have also been common subjects of intelligence studies.
This is very interesting, since the entire bodies of such relatively normal sized birds should not weigh much more than the brains of humans and other higher mammals. Thus "bird brains" might be organized for having exceptionally large intelligence for their sizes. Since birds are descended from dinosaurs, it is possible that some undiscovered species of small dinosaurs evolved human level intelligence sometime before the extinction of he dinosaurs 65 million years ago.
See my post number 32 here: