Output capacitance in analysis of Buck Converter
That answer will depend on the acceptable voltage ripple at the output, and the current draw at the output. A larger capacitor, will result in a lower voltage ripple for a set current. Similarly, a larger capacitor will allow for a larger current draw for a set acceptable ripple voltage.

Depends on you design. A general equation to use is C = I/(f*Vpp).

This site provides a nice explanation with some graphics http://www.skillbank.co.uk/psu/smoothing.htm