Part One: A Naturally Habitable Low Gravity Planet.
The kind of world you need is one with rather low surface gravity to help the giant spiders avoid collapsing under their weight, and high enough escape velocity to retain a breathable atmosphere.
So go to Habitable planets for Man, Stephen H. Dole, 1964: https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf
And go to pages 53 to 58 where he discusses the mass range for planets habitable for humans.
On page 54 Dole decides that if a planet with a surface temperature warm enough for humans can have a maximum temperature of only 1,000 Kelvin in its exosphere, it could retain an oxygen rich atmosphere for long enough if it had an escape velocity of only 6.25 kilometers per second.
According to figure 9 on page 31 which gives the relationship between mass, radius, escape velocity and surface gravity for terrestrial planets, an escape velocity of 6.25 kilometers per second would correspond to a planet with 0.195 Earth Mass, a radius of 0.63 Earth radius, and a surface gravity of 0.49 g.
Earth has an escape velocity of 11.186 kilometers per second so 6.25 kilometers per second is 0.5587 of Earth's escape velocity. Thus Dole's minimum size human habitable world has 0.5587 Earth's escape velocity, 0.49 Earth's surface gravity, 0.63 Earth's radius, and 0.195 of Earth's mass. And those properties don't all change in the same amount when one of them is changed. Each has to be calculated separately for a world of a specified mass and radius.
And that is important because Dole's figure 9 is based on the knowledge of the radii and masses of the terrestrial planets in our solar system in the early 1960s, and we have much more accurate data today, making figure 9 rather obsolete. We even have some data on the masses and radii of terrestrial planets in other star systems.
Thus we know that the average density of terrestrial planets can deviate from that indicated in figure 9. I have noted comparing the surface gravities and escape velocities of worlds in our solar system that when the world has less density than Earth's density, the escape velocity tends to be higher relative to the surface gravity, which is what you want when designing a habitable planet with the least possible surface gravity.
Here is a link to an online surface gravity calculator which I sometimes use:
https://philip-p-ide.uk/doku.php/blog/articles/software/surface_gravity_calc
Here is a link to an online escape velocity calculator which I often use.
https://philip-p-ide.uk/doku.php/blog/articles/software/surface_gravity_calc
The volume of a world relative to Earth varies with the cube of its radius relative to that of Earth (6,371.0 kilometers) and the radius of a world relative to Earth varies with the cube root of its volume relative to Earth. The density of a world relative to Earth's density (5.512 grams per cubic centimeter) varies with its volume and mass relative to Earth.
So you can try designing worlds with low average density compared to Earth to get worlds with low surface gravity to make giant spiders more plausible, and escape velocity high enough to retain atmosphere for geological eras of time.
If you make the average density too low, your planet will have to be totally covered by oceans many times deeper than Earth's, and there will no land for humanoids or giant spiders to walk on. Unless somehow the world has floating islands or continents made of rocks less dense that water.
Part Two: A Terraformed Low gravity Planet.
Or maybe the world is not naturally habitable. Maybe an advanced civilization spent fifty thousand years terraforming that world, even though the atmosphere would be lost into space after only fifty million years. The advanced civilization would have figured that almost fifty million years would long enough for their use. Or maybe they keep on terraforming the world, building giant atmosphere factories which produce new atmosphere at the rate necessary to totally replace the atmosphere in fifty million years.
The Martians in Edgar Rice Burroughs's Martian stories a century ago needed air factories to keep Mars habitable, so that is not exactly a new idea.
Part Three: A Shellworld.
There would be a lower limit to the escape velocity and surface gravity of a world capable of keeping a breathable atmosphere long enough to make terraforming it worthwhile.
So if you want a world with an even lower surface gravity you are going to have to put a solid roof over the world to hold in the atmosphere.
See the concept of a shellworld.
A shellworld13 is any of several types of hypothetical megastructures:
A planet or a planetoid turned into series of concentric matryoshka doll-like layers supported by massive pillars. A shellworld of this type features prominently in Iain M. Banks' novel Matter.
A megastructure consisting of multiple layers of shells suspended above each other by orbital rings supported by hypothetical mass stream technology. This type of shellworld can be theoretically suspended above any type of stellar body, including planets, gas giants, stars and black holes. The most massive type of shellworld could be built around supermassive black holes at the center of galaxies.
An inflated canopy holding high pressure air around an otherwise airless world to create a breathable atmosphere.4 The pressure of the contained air supports the weight of the shell.
Completely hollow shell worlds can also be created on a planetary or larger scale by contained gas alone, also called bubbleworlds or gravitational balloons, as long as the outward pressure from the contained gas balances the gravitational contraction of the entire structure, resulting in no net force on the shell. The scale is limited only by the mass of gas enclosed; the shell can be made of any mundane material. The shell can have an additional atmosphere on the outside.5
https://en.wikipedia.org/wiki/Shellworld
In the last type of shellworld, there would be no surface for humanoids or giant spiders to walk on. The animals would fly with wings or use siphons like squid to move around and wouldn't resempble humanoids or spiders.
Part Four: Moonbases and Space Habitats.
And on a smaller scale, a totally self enclosed colony could exist on an airless low gravity world, or a colony could exist in an artificial space habitat. Nobody now knows the lower limit of gravity necessary for human health, so some planetary enclosed colonies and space habitats might possibly have gravities as low as 0.1 g or 0.01 g for all that we know.
And maybe the giant spiders from Ungoliant III escape from the zoo.
Part Five: Smaller Humanoids Making the Spiders Seem Larger.
Another way to help the giant spiders have humanoid or larger than humanoid size is to make the humanoids small by the standards of Homo sapiens, thus enabling smaller spiders to still be giant compared to the humanoid characters.
In anthropology, pygmy peoples are ethnic groups whose average height is unusually short. The term pygmyism is used to describe the phenotype of endemic short stature (as opposed to disproportionate dwarfism occurring in isolated cases in a population) for populations in which adult men are on average less than 150 cm (4 ft 11 in) tall.1
https://en.wikipedia.org/wiki/Pygmy_peoples
The very short humanoid fossils on Flores are almost universally considered to be a separate species Homo floresiensis
LB1's height is estimated to have been 1.06 m (3 ft 6 in). The height of a second skeleton, LB8, has been estimated at 1.09 m (3 ft 7 in) based on tibial length.4 These estimates are outside the range of normal modern human height and considerably shorter than the average adult height of even the smallest modern humans, such as the Mbenga and Mbuti at 1.5 m (4 ft 11 in),[64] Twa, Semang at 1.37 m (4 ft 6 in) for adult women of the Malay Peninsula,[65] or the Andamanese at also 1.37 m (4 ft 6 in) for adult women.[66] LB1's body mass is estimated to have been 25 kg (55 lb). LB1 and LB8 are also somewhat smaller than the australopithecines, such as Lucy, from three million years ago, not previously thought to have expanded beyond Africa. Thus, LB1 and LB8 may be the shortest and smallest members of the extended human group discovered thus far.5
https://en.wikipedia.org/wiki/Homo_floresiensis
Your story might happen on an island where insular dwarfism has made the people smaller than elsewhere on the planet (and they may have legends of giants living in other places) and where insular giantism has made the spiders larger than elsewhere on the planet.
https://en.wikipedia.org/wiki/Insular_dwarfism
https://en.wikipedia.org/wiki/Island_gigantism
Part Six: Child Protagonists.
And of course there are many examples in human history of children travelling through the wilderness without adult supervision and facing the small but real possibility of encountering dangerous predators. That is another way to make the size of the spider's relative to the characters plausible.
