Rectangular Galvalume Downspouts are the standard for Galvalume 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.

Rectangular Galvalume Downspouts are made with steel that is coated with a material that is 55% aluminum and 45% zinc. This coating is applied to the steel sheet by a continuous hot dipping process. The result is a strong product that is both durable against physical damage and resistant to rust and corrosion. The coating shields steel from moisture and contaminants while providing protection from scratches and cut edges. 

Specifications:

2x3 = 26 Gauge, 10 ft. lengths

3x4 = 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 = 600 sq. ft. of roof

3x4 = 1200 sq. ft. of roof