The largest terrestrial arthropods are the coconut crabs :
Upper size limits for these leggy hefty critters – close to ten pounds, and a bit over three feet in legspread. Pretty impressive!
I’m wondering – is there any particular reason that spiders can’t grow to this size? Is there something different about crustaceans that allows for bigger size then their arachnid cousins?
For that matter, have they ever found a super-massive fossil spider?
Just curious. Don’t need answer fast.
The spider’s legs would need to be far more massive in proportion to support the extra mass.
Square Cube Law.
Crustacean legs are powered by muscles anchored within the exoskeleton. Arachnid legs are hydraulic. One may not scale the same as the other.
Crustaceans mineralize the chitin in their exoskeleton with calcium carbonate, giving it far more mechanical strength than chitin alone. I doubt that an exoskeleton of pure chitin like a spider’s would be strong enough to support the weight.
The largest fossilized spider has a body length under an inch… you can buy a bigger pet spider today. While some other life forms had giant predecessors, it seems spiders never did get that big… in fact the biggest ones we know of are alive today.
I dunno if that can be stated definitively - creatures like spiders fossilize only rarely. It could be the case that larger ones existed, but have not been discovered yet. Note that in your link, until recently, there were only two spiders from the period known. Now there are a few hundred, but all from the same place - who knows how representative these were of spiders worldwide and throughout the period.
The male Goliath Bird-Eating Spider is the biggest at 11 inches, according to Guinness, but according to Wiki, the largest by size instead of mass is the Giant huntsman spider, which can be a foot across.
The fossile Megarachne was thought to be a giant spider, larger than anything found today, and well into coconut crab territory;
nowadays this fossil is thought to be a eurypterid, an extinct scorpion-like creature. Eurypterids grew even larger than this, but they were semi-aquatic, so are more like coconut crabs in some ways.
I don’t think the use of a hydraulic-actuated locomotion is really the limiting factor. Although muscles are preferred by many classes invertebrates and all vertebrate for locomotion and manipulation, there is no physical reason that a hydraulic powered system scaled to size could not work efficiently, up to the tensile limits of the body material forming the hydralic chambers. Our largest machines use hydraulic actuators, albeit often running at 3000 psi or higher operating pressures. However, musculature may be more efficient for long range locomotion, and may be more adaptable from an evolutionary sense, hence why most large animals use musculature for locomotion and manipulation.
In short, I think it is theoretically possible to have much larger spiders and huntsmen than we see today, but evolution seems to have favored smaller body sizes. I suspect the limiting factor has nothing to do with physical strength but the open circulatory system (and to an extent, the use of less effiicent hemocyanin as an oxygen carrier instead of hemoglobin, although this hasn’t prevented the evolution of giant cephalopods) which limits the size of arthropods in general to the amount of oxygen which can be conveyed througout the body.
Stranger
I interpreted that statement differently. 
To me, it suggests that such large spiders shouldn’t exist at all. 