Reminds me of the movie Mimic. What would we have to change for roaches to grow to be man-sized?

If I remember right, the movie scientists were mainly concerned about how the critters would get enough air.

http://www.saburchill.com/chapters/chap0021.html

Natural history suggests the same, long before us when the air was a lot more oxygen rich, bugs were a lot bigger!

http://en.wikipedia.org/wiki/Meganeura