I carry one of these two knives pretty much all the time: The CRKT Crawford/Casper, and the Smith and Wesson SWAT framelock tactical.
http://www.ajaxgrips.com/0/ajax/colu...ges/01/116.jpg
http://www.theknifeconnection.com/in...OD&ProdID=3701
With a 3.7" blade, the smith and wesson is almost too large to carry, because it's just unneccesarily large. I honestly can't imagine carrying a knife larger than that and actually having a use for it. The CRKT knife is a good length for cutting boxes and whatnot, and I suppose if I needed it for defense, it's plenty long enough for that, too. It's not the size that counts, its how you use it

.