A Goliath spider commonly described as "puppy-sized" freaked out even the scientist who found it: "I couldn't understand what I was seeing."
Everyone likes a really big insect, spider, or crab when the film script calls for a monster. From the insectoid MUTOs (massive unidentified terrestrial organisms) in the last Godzilla movie to the claims of a "Crabzilla" photographed off England, giant arthropods things have sparked our fears and imaginations. As a kid, I loved the giant ants in Them! (a good movie, really) and The Deadly Mantis (not that bad a movie, despite being savaged on Mystery Science Theater 3000). Peter Jackson's King Kong remake was crowded with outsized arachnids, which fortunately were vulnerable to being shot away from a person's body by a writer who had never handled a submachine gun before. (All us writers want submachine guns on occasion.)
Such creatures appear in written fiction, of course, as well; Greg Beck recently wrote a pretty good thriller called The First Bird which included spiders big enough to trap and poison humans and centipedes big enough to... umm, you really don't want to know.
The Goliath spider, Theraphosa blondi (Wikimedia Commons)
So, how big can such creatures actually be? The answer is as much in the realm of engineering as in biology. An animal is essentially a machine taking in oxygen and some form of fuel convertible to fats, sugars, etc. It needs enough of these inputs to fuel the digestive system and spin off energy to run all the other processes (growth, mating, thinking, etc. - all the things that make the being autonomous (the word "autotroph" having come into widespread use thanks to The Big Bang Theory.) )
There are limits to practical sizes of all animal types, just like there are of man-made machines.
To look at an artificial example, we can build big tanks, but we couldn't scale a Tiger tank up to the size of a blue whale: the power-to-weight ratios of internal combustion engines and the strength-to-weight ratios of metals are inadequate. (To get picky, you might conceptualize a movable object that size with using a fission reactor and nanomaterials, but it would be a totally different type of machine design: the point is that the tank "order" long ago reached its limits. )
In the natural world, said blue whale, of which giant specimens may push 100 metric tons, is probably the practical limit for all animals, Godzilla notwithstanding. There's only a certain percentage of oxygen in the atmosphere, and mammalian lungs are the most efficient way devised to extract it. The spiracles and book lungs used by arachnids just are not nearly as good, and making them larger doesn't make them better. (The movie Mimic, with its human-sized cockroaches, had the beasts evolve lungs: that film was especially scary to us former Floridians who have seen them almost that big.)
Another killer is the weight of an exoskeleton and the effects of gravity. The square cube law says that, if you double an animal's size in all three dimension, the resulting beast has eight times the weight. The blue whale uses an internal skeleton and the support of water, from which it can't emerge without internal collapse and death. Crabzilla, a photoshop job claimed to be over 15m across, simply couldn't have dragged that massive armored skeleton anywhere even if the respiratory system could support it. A truly awful novel called Spider Legs, by Clifford Pickover, with co-authorship by Piers Anthony, who I'm told was brought in late as a "book doctor," went through all kinds of contortions describing the artificial enhancements (not one of which would have actually worked) by an unscrupulous scientist/breeder to create a monster crustacean.
Anyway, the biggest known arthropod ever to live was a Devonian-age sea scorpion or eurypterid called Jaekelopterus rhenaniae. It may have been 2.5m long and would scare the daylights out of anyone.
Jaekelopterus rhenaniae (Wikimedia Commons)
Today we have to make do with the American lobster (up to 20 kg in exceptional specimens) and, for land-dwellers, the bizarre coconut crab (about 4kg). And we have the Goliath spider, of course.
There are some oddities in the cryptozoological literature. A bizarre animal reported off Florida and nicknamed Specs for its protruding eyes was suggested by zoologist Karl Shuker in a book to be a possible surviving sea scorpion, but there was only one witness and no one has reported it since. Then you have the spiders. There are at least two reports of a "small dog"-sized or larger spider (complete with web) from Papua New Guinea, one of similar-size spiders from Vietnam, a "washtub sized" spider in the United States, and an even bigger species from the Congo. (Shuker has thoughtfully collected these here.)
No matter how you slice the engineering, spiders the size of coffee tables and crabs the size of small yachts, just don't work. But giant arthropods will always be with us in fiction. And they'll always be scary.