Cover as much as you can and can afford. You might want to look into multiple layers of deadner. Some deadners cancel out the frequencies of road noise while others will cancel out other noises such as exhaust. It all depends on the frequencies. The mats will usually deaden a good amount of nose except road noise. A foam layer is made for road noises. check out elemental designs and there line of products. Priced very well.
http://edesignaudio.com/category.php?type=damp Not sure on the square footages because i used liquid sound deadner.