Rectangular Copper Downspouts are the standard for Copper 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:
Gauge = 16oz.
Length = 10 ft.
3x4:
Gauge = 16oz. or 20oz.
Length = 10 ft. or 20 ft.
4x5:
Gauge = 16oz.
Length = 10 ft.
Rectangular 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