Why did insects never evolve (or grow) to sizes more than about 10 to 12 inches long (and, even more strictly no more than an inch or two in diameter)?

See explanation in the Biology forum :)

Anyone know what I can do about this? They are about 3 to 4 millimeters in length and I saw one fly of the wall the other day. They seem to have a black butt. I spray insect spray and they come out dead all over the floor each day but its been going on for a week. Thanks I will try to post a picture. Please email to [email protected] and here. Thanks