Yes, I am well aware of the spiracular limitations of existing insect breathing and blood, but a fictitious mechanism to modify the insects can also adapt their blood and oxygen circulation system to the larger sizes.
Grasshoppers have nice hams - can you imagine herding 1000 pound gigantism adapted grasshoppers? What would you ride? Why Kangaroos - of course...;)
While that exact realization did make me stop being freaked out by giant spiders in movies, technically "they can't breathe" in earth's current atmosphere. Presumably, if it was worth the expense involved, you could grow giant insects in closed biomes with higher oxygen levels and (I assume) higher atmospheric pressure.
Also: Jurassic Park is BS for the same reason. The T Rex would not function in earth's current atmosphere -- at least not without tampering with their physiology and/or some kind of extra support system.
Well, I don't know what the limits would be on size, but you seem to be presuming that historical limits are hard limits. I think that is as much a mistake as assuming current sizes are hard limits. It might take crazy levels of oxygen and pressure or it might be undoable, but if we were to create environments for this purpose, hell, why stick to recreating historic earth conditions?
https://bioteaching.wordpress.com/2012/03/24/gigantism-in-in...