Here's a picture of an 18v drill stripped down. I don't remember if this was a Campbell Hausfeld or a Black and Decker. It is the largest one I have seen so far. The motor on this one is quite long and is much bigger than a 550. The single speed that I used in my Euc is probably 1.5 inches shorter and the one in my KW is somewhere in between. I changed the motor on the single speed because it was only a 540 and spun quite fast. The pinions are pressed on making motor swaps easy if you aren't happy with the speed or torque. Hope this helps.
Reg