You want to cut the footprint about 2 inches less on each side than the perimeter of the tent. Otherwise rain going down the sides of the tent will run right under the tent on top of the footprint.

How much does your footprint weigh?

Most people use either Tyvek or polycro (the kind of plastic used in storm window kits). I haven't used a footprint/ground cloth in years. Silnylon floors are amazingly durable.