Downspout diameters are sized according to the roof area they drain.
The following is the rule of thumb used in the industry:
2x3 inch downspout for 600 sq. ft. of roof
3x4 inch downspout for 1200 sq. ft. of roof
GutterSupply.com has many selections of downspouts in: copper, aluminum, copper clad stainless steel, galvanized, and lead coated copper.