How do I calculate the size of the downspout I need for a gutter?
Size the gutters for the rainfall, but go with the largest downspout that will fit your gutters.  

The issue for downspouts isn't water capacity in a torrential rain, it's debris that gets washed into the downspout.  The larger the downspout, the better debris will wash through.  Large downspouts are harder to clog, pass more water when they do clog, and are easier to clean out.