I am writing a story that has insects the size of humans, but I am having a little trouble with the biology.
When you look up why there are no large insects on Earth, the most popular answer is that Square-Cube Law limits how large an organism their cardio-respiratory system could support. I feel like this is not that much of a problem to solve though. I just bump up the oxygen levels until things look about right and I'm done.
The problem I have is with molting. Each time an insect molts, it only grows a couple of centimeters. It would take decades just for my aliens to reach the size of a baby. Somehow, they need to grow an whole bunch every time they molt, and I cannot think of how.
In addition, for a while after an insect molts its new exoskeleton is much softer than it normally is. It is actually soft enough that if the insect is put into a confined space, its limbs will harden completely misshapen. Unless I make the gravity really low, the alien will not be able to support it how body weight.