If its a speed 400 type motor it would take around 7.4 volts. In the old days NiCad batteries could be connected directly to the motor with no speed control. They would then just run down and the plane would land with no damage to the battery. The battery could then be recharged. LiPOs wont stand a complete discharge so a speed control must be used.
Here are the dimensions of a speed 400 from the Hobbly Lobby Catalog:
SPEED 400 7.2V Electric Motor
For airplanes that weigh up to 25 oz., sailplanes to 45 oz. These are motors that consume from 35 to 75 Watts and about 6 to 10 Amps, have 2.3mm diameter prop shafts, are 27.6mm diameter, about 1-1/2" long, and weigh about 2.3 oz.