The diameter of the biggest part, namely the camera head, is 23mm/0.9in. 

So when it powers off, it goes into a sleep mode. That sleep mode doesn't require alot of power, but if you have it set to turn on if the camera detects vibrations, it uses the cars battery. Mine is plugged in all the time and it only powers on when I turn the car on or it feels vibrations. With the vibration setting, it will turn on when I open my car door. But if your power source isn't powered when the car shuts off, then it doesn't use any power, just what the camera has stored up. Hope that helps 

Yes …. You can use anywhere it goes with you… I use mine in our camper 

Our wireless endoscopic cameras work with cars/vehicles. Such as carbon deposits on piping valves, removing missing bolts in engine bays, inspecting cylinder walls and pistons, gearboxes, muffler pipes, etc. 

I don't know the exact size but it even fits 12-13 year old kids.