Pruning Knockout Roses
"You can prune Knock-Out roses back quite severely when they are dormant in winter. In Eastern Tennessee, Zone 7, late February or early March would be a good time to prune. You can prune all the way back to several inches above the soil line or graft union. Keep in mind that most regrowth will come from within 6 to 12 inches below a pruning cut. So a cut low to the ground will cause the bushes to fill in at the base. If you only trim them back to 3 feet in height, they'll branch there, and be bare at the base.
You can also do some shaping pruning during the growing season. If several shoots become much longer than others, trim them back to maintain a more uniform height."