Downspout diameters are sized according to the roof area they drain.
The following is the rule of thumb used in the industry:
2×3 inch downspout for 600 sq. ft. of roof
3×4 inch downspout for 1200 sq. ft. of roof
4×5 inch downspout for 2000 sq. ft. of roof
We also carry a complete line of rectangular elbows, tile adapters, y diverters/y funnels, and the necessary downspout brackets and fasteners.
The typical downspout size for a 5″ K-style gutter is 2×3 inches, but 3×4 inches is preferable because it is less likely to become clogged and is easier to clean out.