Will modern-day insects grow to massive size in a high-oxygen environment?
While @CalendarJ comments "Yep!" above, a quick search suggests that experimental hyperoxia typically only increases insect body sizes slightly. Everything below is from VandenBrooks et al. 2012 (of course there could be other sources that contradict this!)


  A few beetles have been reported to reach larger sizes when reared in hyperoxia, but most of the species examined reach similar body sizes in 40% oxygen as in normoxia (Harrison et al. 2009). 


and:


  Here we investigated the impacts of Paleozoic oxygen levels (12–31%) on the development of Blatella germanica cockroaches. Body size decreased strongly in hypoxia, but was only mildly affected by hyperoxia. 


and this figure:



These results show that male cockroaches grow much larger under present-day (normoxic; 21% oxygen, highlighted box) and elevated (hyperoxic; >= 24% oxygen) levels than under low (anoxic; 12%) conditions, but the hyperoxia-grown roaches are only a bit larger than normoxia-grown ones. (Similar results for females not shown.)



VandenBrooks, John M., Elyse E. Munoz, Michael D. Weed, Colleen F. Ford, Michael A. Harrison, and Jon F. Harrison. “Impacts of Paleo-Oxygen Levels on the Size, Development, Reproduction, and Tracheal Systems of Blatella Germanica.” Evolutionary Biology 39, no. 1 (March 1, 2012): 83–93. https://doi.org/10.1007/s11692-011-9138-3.