When I'm in the city working its always been a small knife. My Spyderco Mouse, Dragonfly, or at the largest something like my small sebenza (which actually gets carried the most). At home on the range, I'm into fixed blades, but here too, I've noticed that smaller works just fine.
As I perform various cutting chores around the farm, I keep asking myself how small could I go and still do this job conveniently? The answer is, pretty small, maybe 2"-3" is the most I usually need. The exception is in food prep. There I've noticed that even a 3" blade is a little small for convenience much of the time. 5" works well here, but too obvious on my belt when I wander off into town and forget to change knives. I've discovered that 4" seems to be about right. I can do all the cooking I usually do with much larger kitchen knives. Its small enough to work with easily for all those other chores that could just as well be done with a 2" blade, and most folks don't freak out if I happen to have it on my belt when I go to the store or something (this is (or was) a farming town after all)...