To install a downspout extension properly, measure the outlet located at the bottom of your downspout. This is usually going to be two by three or three by four inches. Sometimes you may find a three inch round or a four inch round downspout. These products are easy to confuse when shopping for them, but you can find them in these traditional sizes. Usually there is not much assembly required. Most of these products are preassembled, snapped together, or made out of just one piece.