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Fantasy and folklore has many instances of giant spiders,ranging from creatures like the J'ba Fofi to Aragog from Harry Potter. My question intends to ask whether a creature like the J'ba Fofi could have evolved.

On a planet with conditions identical to Earth, how could a giant spider with a leg-span of 1 meter evolve? Could it produce silk and how much could it produce if it could? Could it walk without violating the Square Cube Law? How could it breathe?

This question is part of the Anatomically Correct Series.

I am going to reuse parts of my answer to Anatomically correct Arachne, which is related but not a duplicate.

We cannot approach this as if the only way for this creature to exist to exist is in the form of a giant nope spider thing without any modifications to its spider anatomy. That would be silly.

We all evolved from very small animals that lived underwater, yet here we are in human form. A much larger stretch than having arthropods growing into big sizes.

Specially because in prehistoric times, we did have arthropods that were quite big. The largest arachnid ever was a 70 cm long scorpion. That's a little over two feet, or 28 inches. It had aquatic cousins that could grow up to 2.5 meters long (that's over eight feet), though. Granted, those beasts had their weight supported by water, but look at that... An arthropod larger than a man! Not only that, but we know that there's less oxygen available in water than in air, so how could the square-cube law allow for that?

Well, besides evidences that our atmosphere was more rich in oxygen millions of years ago, there is also the fact that those arthropods had evolved their internal anatomy to allow for those sizes.

Let's go back to the creature in the quedtion then.

Imagine if you will a prehistoric nope tarantula. It lives in a world where there is more oxygen available than Earth right now, because that's how prehistoric Earth was like. Our prehistoric nope spider is under many different evolutionary pressures, and a larger size is conferring it increased rates of survivability and reproduction.

To allow for that, the nope tarantula starts developing, throughout generations, some unusual characteristics for a spider. For example, a mutation gives it layers of skeleton, so besides the external chitinous one, the spider has internal ones as well. Over time the external skeleton is replaced with soft but thick leather. This allows it to replace its book lungs with mammal (or bird)-like lungs, which are much more efficient.

Over time, as this species of nope tarantula grows, it will evolve a circulatory system much like a worm's at first, then that of a vertebrate.

As for the abdomen, it resembles that of a reptile or amphibian more and more, with different organs going through convergent evolution to take the roles of a liver, a spleen, a pancreas and so on. Nopes spiders have excretory organs in their legs, but this species's excretory organs move to the abdomen over time.

Now, nopes spiders are known for being very agile with their legs, and weaving webs with them. Our specific nope spider, though, is growing larger by the millenia, so its six frontmost feet are only used for supporting weight now. Eventually they will be completely flat and fingerless (yes, nopes spiders have fingers - actually microscopic claws, which is how they weave and cling to stuff). Only the last pair of legs will have fingers for weaving.

In the other question I linked to, Azuaron suggested that a giant nope spider-like creature would have to have its legs below the body and not splayed. I propose a midterm solution which is close to the anatomy of a real nope spider, so the legs should be fixed below the cephalothorax. The creature is supported by eight feet, and they don't have to be incredibly heavy for their size, so I think this setup works.

I estimate that, once the creature reaches its adult size, it should weight 60 kilograms (approximately 132 pounds), with its weight divided almost evenly among its eight feet (the hindermost feet would support a smaller fraction of their weight), so on average each foot would be supporting 7.5 kg / 15.5 pounds.

Such a nope would probably be warm-blooded, with all the adaptations required for that. It should eat at least as much as a human with the same weight.

As for the web output, I think it would be proportional to its body mass, relative to any regular species of web weaving spider.