Small is Scary
In traditional fantasy role-playing games, everything is big. You thought a centipede is scary, we made it 10 feet long. That regular spider freaking you out? Here's one 5 feet tall! It doesn't really matter what it is, the rule is to make it as big (or bigger) than a person, and maybe cover it in spikes and/or bony plates.
In reality though, the bigger you make a thing, once you get over the shock factor, makes it a lot more manageable. A giant spider could cause some initial bladder control issues, but once you get over that it's a big-ass spider, it's not any different than the any other wilderness or dungeon denizen. You square up, bust out the spells, spears, and swords, and have at it, and because it's fairly large, a group can gang up on it and have a fairly easy time of hitting it, because of it's size, the tactics required are pretty straight forward.
What is really scary is the small ones, especially if you make them lethal, community web building hunters. Imagine, the trees or dungeon surfaces are wrapped in a shroud of webbing, in which tens of thousands of spiders no bigger than a large common spider (not even a tarantula) all armed with necrotic, paralyzing poison. They trail 'hook lines' down into corridors, and as passing prey connects with the slight, sticky filaments, they dislodge the spider from it's ceiling roost, where it quickly finds it's way into the warm inner folds of armor and garments, where it delivers numbing bites as it roams along.. eventually, the prey will fall as their muscles seize up, their heart barely beating, and numerous small spiders emerge from the webbing to wrap the nigh-corpse in a it's very own death-shroud to preserve it's bodily fluids, as the poor sap becomes a larder for all those hungry little monsters, poison softening the tissues of the body into a delicious soup.
How do you combat that?
Burn the webs! but alas, while direct application of flame will cause them to shrivel and singe, like many webs, they simply refuse to ignite... so one must go along and inch by inch burn the webs, but even then, you haven't destroyed the fiends, only the time-crafted lair that made their job easier. Purging expanses with fireballs and other expansive pyrotechnics can make quick work of the lair, but so many of the spiders are still in their tunneled warrens, pocking the walls, ceiling, and floors of their domain.The drag lines will still be lowered, the victims still claimed.
You can't very well fight them with sword or spear... mallets and blunt flails maybe, but even then, those weapons will take horrible abuse to eradicate so many minute foes. And even if we made these fiends the size of grapefruit, sword and spear would be blunted and dulled clashing against the stones while slaying them, but alas, they are the size of small nuts, and even harder to hit. Stomping on those you can reach may work well at first, but that may simply expose you to more.
The greatest plate armor in the world, besides being impractical gear for dungeon delving (imo) would truly show itself as a liability against these foes... mighty warriors squealing like school-children, slapping at their face and hair, ripping armor off for fear of a spider in their gusset.
It's the smallest monsters that you need to be scared of.
Inspired in part by this tale of RL drain exploration. And this story of a house infestation.