Insects and other land-based Arthropoda are stuck at the size they are due to how inefficient their respiratory system is. They were able to reach the size they did in the Carboniferous period because of the incredibly high oxygen content in the atmosphere back then.
Let’s say we were to, through genetic engineering, give an insect a more efficient respiratory system, like a pair of mammalian lungs or even avian lungs, then released it into the wild. Then, we left it on its own to evolve.
Now that it has room to grow, how big could it possibly get? Does the arthropod body plan (i.e. an exoskeleton) have certain advantages or disadvantages? Could they get as big as mammals, or would they be stuck at a certain size? What would they look like?