This is about as small as you will ever need. These tiny motors are commonly referred to as N20 geared motors. They have a 3mm shaft, but that's easily fixed with a 4mm brass sleeve. The mounting bolts are M1.6 and are so small it's difficult to see them let alone get them on the end of a screwdriver to bolt the motor to the bracket. I drilled 2mm holes in the part 11 double bracket as this allows the countersunk heads to fit nice and flush. Running on 12 volts these motors are quite difficult to stop by hand. Be careful not to damage them in a plastic jaw vice as the gears protude ever so slightly and I mangled my first one by holding it in the vice while fitting the brass sleeve.