This Electric micro motor is only 6mm diameter for better use of space allocation. 

Voltage: 3vdc
• RPM: 1200
• Torque at stall: 0.28oz.in (0.02 kg-cm)
• Current: 100mA
• Length: 20 mm
• Diameter: 6mm
• Weight: only 1.2 grams!

Looking for a tiny gear motor? Only from Firgelli comes the GM6 Planetary Gear Motor with a straight shaft that you can actually use, smaller than many pager motors! The output shaft is 1.5mm diameter with a flat on the side.

We're quite excited about these motors; they're quality made, inexpensive, and tiny! We expect to see lots of nano-sumo robots built out of these mini micro motors