It's about an inch (2cm) long, was flying around our bedroom, and was tough as nails. I have one of these* (basically a tennis racket with a charge running through horizontal wires), and every time I hit it there was a loud crack, a blue flash, and it _still_ kept going for three hits.
If I'm likely to have a whole nest of immortal massive insects in my walls, I'd like as much information on what they might be as possible...
(Only one so far, so it's entirely possible it came in through a window a couple of days ago and snoozed until five minutes ago. I'm certainly hoping so!)
*The reviews are awesome. People clearly get the urge to kill after getting one.
Original post on Dreamwidth - there are comments there.