Yeah, but a human sized roach has an armoured carapace which is now a few centimetres thick. Do you have a cannon with armour piercing ammunition at home? I'll take my chances in forming an alliance with the small people.
A roach that big wouldn't be able to move much, it's legs would probably splinter under the weight of that armored carapace. Also it probably wouldn't be able to "breathe" at that size, it's respiratory system wouldn't function well enough to keep it alive for very long. It's a problem that would solve itself pretty quickly.
Definitely the 1 human sized roach this one's obvious. It's hard enough getting rid of 1000 roach sized roaches and now you want to give them human ingenuity?
Human sized roach. I mean Gregor just kinda died of his own accord didn't he? You just gotta emotionally reject it as having ever been your son and maybe throw an apple or two at it.