These small brushed DC gearmotors can deliver a lot of power for their size. This version has a 6V brushed DC motor combined with a 78.13:1 metal spur gearbox. The gearmotor is cylindrical with a diameter of 20 mm, and the D-shaped output shaft is 4 mm in diameter and extends 18 mm from the face plate of the gearbox.
Key specs at 6 V: 190 RPM and 170 mA with no load, 62 oz-in (4.5 kg-cm) and 2.9 A at stall.
More technical information at Pololu.