I don't know if there is a standard calculation that builders use for this -
But you should be able to figure it out by taking the square footage of the roof (just the area that will be draining into the gutter in question) and multiplying it by the maximum expected rainfall per minute - that will give you the volume of water per minute that the gutter/downspout needs to handle.
Then measure the gutter (length, width height) to get its volume. The downspout volume will be length times 3.14 (pi) times the radius-squared (1/2 the diameter, squared.)
As I said, I'm just guessing here - so how to interpret this data? - use your judgement - can a gutter/downspout combination with a volume of X cubic inches handle the water flow of Y cubic inches per minute?