There are a few ways to select the proper value of output capacitor (COUT). The output capacitor value can be selected based on output voltage ripple, output overshoot, or undershoot due to load transient.
The ripple current rating of the output capacitors must be enough to handle the output ripple current. By using multiple output capacitors, the ripple current can be split. In practice, ceramic capacitors are placed closer to the diode and the MOSFET than the bulk aluminum capacitors in order to absorb the majority of the ripple current.