For combat I would lean toward shorter than the Tali-whacker in length. If it is for duty, it needs to be on your belt and a 7 inch (or plus) blade is more than you really want to carry because of getting in and out of vehicles and up and down on the ground. Stick to something NO LONGER than 6 inches. It will be a utility knife MUCH MUCH more than a defensive/offensive blade.
BUT as is said, and is very true, to each his own. If that is what you want, that is what you want.