So, summing up: how big can they grow? Depends a lot on the environment they live in, but with the proper oxygen levels, they can be at least as big as a small crow. Can they have moderate intelligence? Very likely, as they have not only good oxygenation of nervous tissue, but various ganglia across their bodies handling vital functions, which could allow their main brains to specialize further in order to grant them moderate intelligence. Though I don't think they'd have the complexity of our brains, they'd likely be capable of making long and short term plans, making ambushes and traps and living in a community at the very least, as most of these abilities can already be seen to some extent amount various arthropods. Overall, their larger size would also allow for larger brains and therefore greater potential at becoming smarter, but I don't find it likely that they'd be too engaged in things such as tool using.

So, summing up: how big can they grow? Depends a lot on the environment they live in, but with the proper oxygen levels, they can be at least as big as a small crow. Can they have moderate intelligence? Very likely, as they have not only good oxygenation of nervous tissue, but various ganglia across their bodies handling vital functions, which could allow their main brains to specialize further in order to grant them moderate intelligence. Though I don't think they'd have the complexity of our brains, they'd likely be capable of making long and short term plans, making ambushes and traps, understanding abstract concepts and symbols and living in a community at the very least, as most of these abilities can already be seen to some extent amount various arthropods. Overall, their larger size would also allow for larger brains and therefore greater potential at becoming smarter, but I don't find it likely that they'd be too engaged in things such as tool using, not. Meaning however that there's no chance that they could.

Now, regarding the brain. Dragonflies, with their independent wing control (they're each controlled separately basically, which grants them their great maneuverability) and complex eyes already indicate more well developed brains. However, it'd be very hard for an insect to develop complex thinking like we do, as their brains are much simpler and smaller, to the point that many insects can live for several days after loosing their heads, even more if they were still able to feed /eat without them. That is not to say, however, that they're dumb, as they've been shown to be able to plan and memorize. They will need to go through a decent level of development to reach a more complex level that enables intelligence though, based on our own insects. The presence of a pack behavior could help reach such levels, as well as some level of emotion and empathy (insects as we know them are completely incapable of emotion or empathy, meaning that unless your race develops these abilities, they'll be essentially a race of psychopaths by our standards, with little to no concern for one another, unless they develop some sort of instinct to protect the other members of its species).

So, summing up: how big can they grow? Depends a lot on the environment they live in, but with the proper oxygen levels, they can be at least as big as a small crow. Can they have moderate intelligence? Very likely, as they have not only good oxygenation of nervous tissue, but various ganglia across their bodies handling vital functions, which could allow their main brains to specialize further in order to grant them moderate intelligence. Though I don't think they'd have the complexity of our brains, they'd likely be able to understand concepts such as passage of time, making long and short term plans and living in a community, as most of these thought patterns (except time perception other than day/night) can be seen to some extent amount various arthropods.

Now, regarding the brain. Dragonflies, with their independent wing control (they're each controlled separately basically, which grants them their great maneuverability) and complex eyes already indicate more well developed brains. However, it'd be very hard for an insect to develop complex thinking like we see in humans, as their brains are much simpler and smaller. That is not to say, however, that they're dumb, as they've been shown to be able to plan and memorize. They will need to go through a decent level of development to reach a more complex level that enables intelligence though, based on our own insects. The presence of a pack behavior could help reach such levels, as well as some level of emotion and empathy (insects as we know them are completely incapable of emotion or empathy, meaning that unless your race develops these abilities, they'll be essentially a race of psychopaths by our standards, with little to no concern for one another, unless they develop some sort of instinct to protect the other members of its species). Additionally, they do have the advantage of having ganglia that control vital functions, meaning that their main brains could, without too much of a stretch, become larger and more specialized to handle higher functions such as more complex thinking and understanding complex concepts, such as the passage of time and the awareness of their death. I recommend taking a look at this to get a better understanding of how insect brains work. It also strengthens my point that they'd likely have evolved from a group of arthropod-like pack hunters.

So, summing up: how big can they grow? Depends a lot on the environment they live in, but with the proper oxygen levels, they can be at least as big as a small crow. Can they have moderate intelligence? Very likely, as they have not only good oxygenation of nervous tissue, but various ganglia across their bodies handling vital functions, which could allow their main brains to specialize further in order to grant them moderate intelligence. Though I don't think they'd have the complexity of our brains, they'd likely be capable of making long and short term plans, making ambushes and traps and living in a community at the very least, as most of these abilities can already be seen to some extent amount various arthropods. Overall, their larger size would also allow for larger brains and therefore greater potential at becoming smarter, but I don't find it likely that they'd be too engaged in things such as tool using.