A couple of key points of clarification,
The square-cube law generally governs the maximum size that animals can attain largely due to structural strength of their bones. You need to factor in sheer forces in essentially tubular-shaped bones when calculating when a volume of mass exceeds the structural capacity of its bones to support it.
Kleiber's Law governs an organism's metabolic rate as it increases in size scaling as a factor of 3/4 of the size increase. A mouse 100 times the size of a normal mouse would not have a metabolism 100 times as great as the normal mouse's.
Insect respiratory systems are something on the order of 200 thousand and 10 thousand times more efficient than human respiratory systems in the exchange of O2 and CO2, respectively, than mammalian respiratory systems are. Current O2 levels in our atmosphere do, indeed, limit the maximum size of insects currently, but it is nowhere near their current size. Studies have shown that larger insects are certainly possible, as much as 30-35% larger on average before problems begin to occur. CO2 levels, because the exchange rate of getting rid of the CO2 is much lower than the exchange rate of O2, are much more critical.
The real limiting factor in insects is their internal body mass to respiratory system volume ratios. As the body mass increases insects acquire "constriction points" in their internal anatomy that prevent additional expansion of their respiratory systems to transport O2/CO2 to/from the extremities. The limitation is somewhere in to 25-35% increased size area, right about the point where sheer stresses start to play an important point in how much larger they can become. In addition, and maybe not coincidentally, this is about the estimated oxygen concentrations for periods when giant insects historically occurred.
I researched this subject extensively for my book Hatchings and more than a few entomology PhDs found the science in the book sound.