NEMA is the only motor size/configuration standard that I've found so far. NEMA 8 is the smallest I've found yet. I'm curious what you found. I've noticed a few things on motors, decreasing weight decreases torque precipitously. I couldn't find anything below 120grams that I thought was worth using, the amount of torque generated per unit weight went down too far. Maybe you've found something goby JRDM - Stepper Motors, Servo Motors, DC Motors