The "Bolsover Dragonfly" is a famous eponymous fossil from the coalfields of Derbyshire. Furthermore, "Megaranea", sometimes categorised as "Megarachne" was a contemporary (that is to say, carboniferous era) spider, up to 2 feet across, or about 600mm.
The reason for these very large (what you would call as an engineer, "normally-aspirated" animals) was that the Oxygen percentage of the earth's atmosphere is known to have reached 36% at that time. Therefore very very large insect-type creatures would have been able to respire and exist. This also explains the prevalence of highly-carbonised coals from the time, consisting of very high % levels of pure carbon rather like coke, probably due to continent-sized forest fires which oculd have been started by lighting even in torrentialo downpours.
I would not have wanted to be a land animal 320 million years ago at all.

The hell with big bugs - I want 'em to clone mammoths. I want mammoth steak for tea, a mammoth-skin rug on the floor. I want to run a mammoth ranch in Canada and use my mammoths for logging - just like they use elephants in India. It would be grand! You can all come up and stay in the lodge and go on a mammoth safari - that's gonna be a side business. Oh yeah, and the ivory, definitely the ivory. Have to open a gift shop too.

due to global warming they were forced to decrease their surface to volume ratio so they could better radiate heat....

increase

global warming, anyway

Animals have been around for about 600Ma. The lifespan of a species is around 0.5Ma. Sizes go up and down, depending on the climate, mix of predators and prey and other factors. It would be incredibly surprising if there were not larger species from most groups at some stage in the past.

steve, the debate is over - the Goracle has spoken. it doesn't matter what we find. big ,small. It's global warming

Fact: global warming will cause giant spiders to eat you alive. Tomorrow, probably.

You'd go bankrupt Nick M
Mammoths are completely untrainable
Take it from me., and you should see what they've done to my hall carpet!
The wool is great though. We've insulated the loft with it.

The scorpion was aquatic, so had a different breathing mechanism compared to dragonflies, which use spiracles. It had gills. A land encounter for such a beastie is another thing entirely.

Land Scorpions (or at least some) and primitive (and often the larger) spiders use book lungs, evolved from book gills. I am unclear if book lungs were phased out because they desiccated more than spiracles and were unnecessary in the high oxygen content at the time, though it makes sense. Maybe burrowing spiders, scorpions could reduce desiccation and so the advantage of moving to spiracles was reduced for them.

I am unclear how good book lungs are vs spiracles, but they seem a better starting position for creating forced respiration and thus the chance for massive arachnids.

Can't help thinking those mammoths must have whiffed a bit though, NickM and RAB.

That's one of the great advantages of living in a 21st Century city: no animal smells, and thanks to our marvelous chemical and toiletries industries, precious few human ones too.

(PS: I've just had the same Turing code three times in a row. Something up?)

Sam,
Which is why the place would be staffed by cloned Neanderthals. Think of me as a paleolithic Willy Wonka.

I think there are also issue with chitin strength/exoskeleton mechanics for large arthropods, particularly in the joints. The sea scorpion is at least partly aquatic, which helps.

An 8-foot long arachnid?

It must be either Ungoliant or Shelob.