Also, hasn't the link between hyperoxia and arthropod size been only partially-established experimentally?
I have no idea how quickly you could establish selective pressure towards larger sizes...
And it's not like we don't have massive terrestrial arthropods today with our middling O2 levels, it's just that they're rather rare: http://www.jaunted.com/files/16133/Coconut_Crab_385.jpg
But that's a crab. It doesn't breathe the way insects do. Insects are (probably) limited by diffusion and thus by O2 levels. Crabs, and spiders for that matter, aren't.