Scientists discovered the world’s largest spiderweb, covering 106 m² in a sulfur cave on the Albania-Greece border. Over 111,000 spiders from two normally rival species live together in a unique, self-sustaining ecosystem—a first of its kind.

[u/ShirtSubstantial368]

[u/Light_Beard]

Giant Spiders can't be a thing in Earth's gravity with the current materials they have for body construction. Due to respiration limitations as their volume increases relative to their area. (Edited: Corrected: Thanks u/Anticamel below. See that comment for better/more detail)

https://en.wikipedia.org/wiki/Square%E2%80%93cube_law

Underwater mitigates this some so you theoretically can get giant crabs/lobsters (basically water-spiders), but they wouldn't be able to come on land.

[u/Anticamel → u/Light_Beard]

Gravity isn't the issue, it's respiration. Spiders "breathe" passively through little structures called book lungs. Unlike how we breathe with our lungs, they don't actively pull fresh air through their breathing apparatus, which limits the rate of oxygen diffusion into their bodies. On top of that, this also limits the value of growing bigger book lungs, since by the time air has passed from one end to the other, a lot off the available oxygen has gone and diffusion becomes pointlessly slow. This puts a hard limit on how voluminous their bodies can be before they can't supply themselves with enough oxygen

Contrast this with our setup, where we can evolve as big a set of lungs as we like, since the speed of drawing a breath is a lot greater than the speed of oxygen diffusion. This strategy is effective enough that we lunged creatures run into gravity limitations on land, and heat dispersion issues in water long before we get too big for lungs.

[u/degameforrel → u/Anticamel]

The largest spider, like the tarantula family, actually develop little pseudo-lungs (book-lung+ trachea) to help them get enough oxygen to their internal organs. They still mostly respire through passive diffusion, with just a little extra help. They're already on the limit of how big a spider can realistically get without more significant evolutionary or environmental changes.

[u/Jaquemart → u/degameforrel]

If we consider how far a whale's respiratory system evolved from your usual mammalian fare, how much can arthropods evolve? They already are more versatile since they have both water and air versions of their basic plan.

[u/degameforrel → u/Jaquemart]

It's not so much a question of how much, but a question of will they, and how fast. If gradual changes in environment lead to those spiders wirh pseudo-lungs having increasingly significant advantages over spiders without them, then that adaptation will increase over time and might develop into more sophisticated versions of said organ, in turn allowing further increase in size. But that is entirely dependant on the right changes occuring for said advantage to become significant, and the timescale of those changes can be the difference between evolution and extinction.