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.
Specifications:
2x3 = 26 Gauge, 10 ft. lengths
3x4 = 26 Gauge, 10 ft. lengths
4x5 = 26 Gauge, 10 ft. lengths
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