Bulk Capacitor for Motor Driver
The bulk capacitor makes sure the supply voltage and/or motor voltage doesn't drop too much in case of current spikes/surges. The capacitors will not contribute for the DC current drawn by the motors.  

You should find out what the inrush current / surge current is. For DC motors the inrush current equals the stall current.  

Decide what voltage drop you want and approach the capacitor size with 
$$ C = \frac{ i_{inrush} \cdot t_\text{duration on inrush current} }{ V_{drop} }$$.