Well the most common use is in a solaroller which is a small wheeled car that charges up and then moves in a quick burst of speed. Since you can only use motors and coils with this circuit robots built with this circuit are mainly mechanical such as my artistic butterflies. One other common use is a little thing called a symet which can roam around and not get stuck using only one motor.
Choosing the right size capacitor First you need to think about what your application is. Do you want the motor to spin for a long time or just a a few turns? If you want it to spin for a long time then your going to need a bigger capacitor. I mean bigger