The main limitation to insect size is gas exchange. An insect does not possess any lungs or gills; instead it possesses various holes (spiracles) on the outside of its body, which lead into tracheae that branch out all over the body and supply oxygen directly to the animal's tissues. Insects don't have blood in the traditional sense, because they don't need it, thanks to the fact that gasses just diffuse right into their cells instead of having to be diffused into the blood first.
Now here's the kicker: in places where the exoskeleton pinches and becomes narrow (i.e: joints on the extremities), the trachea still needs to pass through. In smaller insects, this is not so much of a problem, since their tracheae don't take up much space and can be very slim; able to pass through their tiny joints. As an insect gets larger, however, the trachea needs to grow disproportionately large to accommodate gas exchange into the extremity. Once the trachea takes up 90% of the space within the joint, the insect cannot physically grow any bigger, because it still needs to fit in things like tendons and ligaments. For an insect to grow to human-size or larger in our type of atmosphere (low oxygen density), it would need to change its entire respiratory and circulatory system. Doing so would render the question moot, since if it were to alter its biology at such a fundamental level, it wouldn't really be an insect any more.