1000uF was just a suggestion. You can use a lower value, but the ripple current rating should be at least half the maximum motor current. Maybe something like http://uk.farnell.com/rubycon/10tzv470m8x10-5/capacitor-smd-470uf-10v/dp/1281858RL. The purpose of the capacitor is to reduce noise on the power and ground lines when you PWM the motor.