The giant dragonflies of ancient Earth with wingspans of up to 28 inches are generally attributed to higher oxygen atmospheric levels in the atmosphere in the past. New experiments in raising modern insects in various oxygen-enriched atmospheres have confirmed that dragonflies grow bigger with more oxygen, or hyperoxia. [...] The team raised cockroaches, dragonflies, grasshoppers, meal worms, beetles and other insects in atmospheres containing different amounts of oxygen to see if there were any effects. One result was that dragonflies grew faster into bigger adults in hyperoxia.
"The dragonflies were the most challenging of the insects to raise," said VandenBrooks because, among other things, there is no such thing as dragonfly chow. As juveniles they need to hunt live prey and in fact undergraduate students had to resort to hand feeding the dragonflies daily. "Dragonflies are notoriously difficult to rear," said VandenBrooks. "We are one of the only groups to successfully rear them to adulthood under laboratory conditions."
Sadly, it sounds like they didn't actually grow giant dragonflies, they just got dragonflies that grew to adult-size faster. But this is an important first step!