Determining the input capacitance for a SMPS
There are a couple of things to keep in mind when selecting input capacitors.  If the input caps are electrolytic, you must check the ripple current to be sure you're not exceeding the rating.  Then, you should check the amount of ripple voltage you will be inducing on the input voltage and make sure you have enough capacitance for your requirement.  Typically you should have enough capacitance to keep the input ripple voltage below 5%.  

For ceramic capacitors the ESR is low enough that you don't need to worry about the ripple current rating, so target enough capacitance to keep the input ripple voltage where you want it.  Be sure to take the voltage and temperature derating of the caps in mind since ceramic caps with high volumetric efficiency can have terrible derating characteristics with applied DC voltage and temperature.

This is all especially important on the input to a buck converter because the input current is discontinuous, resulting in lots of input ripple current. (Likewise on the output of a boost converter.)

Here's a link to an article that discusses the equations and specifics of how to select the capacitors and capacitance:

Selecting input capacitors