Hey. Never made a spinner but I've seen lots, so take what I say with a pinch of salt. You'll want a brushless motor for your spinner, something with a high kv rating if you're going for massive RPM on the bar. Probably an outrunner like a helidrive or equivalent. Have a search to see what I mean, but make sure you check all your stats before you buy anything. As far as connecting the motor to the bar, every spinner I've seen uses a belt, usually with teeth, and pulleys that lower the speed of the motor a little (small one on the motor shaft, larger one on the weapon). I assume this is so that they slip under shock, and don't just strip like gear teeth. Remember you'll also need some way to adjust belt tension. As for the balancing and dimensions, I don't know enough to help with that. :P