Rectangular Galvanized Downspouts are the standard for Galvanized K Style gutter systems. 2x3" are typical for a 5" K Style gutter system, however 3x4" can also be used and are considered a better alternative due to the larger amount of rainfall they can drain. 3x4 are typical for a 6" K Style gutter system and 4x5 for larger roof areas and commercial applications.
Downspout diameters are sized according to the roof area they drain.
The following is the general rule used in the industry:
2x3 inch downspout for 600 sq. ft. of roof
3x4 inch downspout for 1200 sq. ft. of roof
4x5 inch downspout for 2000 sq. ft. of roof