alt Hacker NewsFrom what I read recently (and I don't remember where it was), the current thinking is that it wasn't oxygen levels or temperatures, but the lack of predators that let dragonflies grow that big. A big dragonfly is much slower and an easier target. So unless you get rid of birds, you won't have giant dragonflies.
You need high oxygen content in the air though. Insect style circulatory systems aren't efficient enough to get oxygen to the cells without the air having a super high concentration of oxygen to begin with.
Basically like how when people can't breath good you put them on oxygen to keep them alive only getting oxygen into the blood is the bottleneck rather than into the body.