My most used knives tend to be 4 inches and under.
Most of the time I use nothing more than my Izula when out and about, although I'm counting work/farm use in with this. If my Izula can handle anything I do on the farm, it'll handle anything I would want to do in the woods, save be a practical butchering knife for medium-large game (I wouldn't be afraid to do a whole deer with one, but it would get tedious after a while).
When the spectre of larger work (woodworking/carving projects, game processing) is in view I carry something closer to 4 inches.
I have found little use, personally, for knives in the 6-8 inch ish range. Just got an Junglas about a month ago, and I think that knife will prove to have some usefulness for me, but I have not been able to use it very much yet. It will probably make a great all-around camp tool/brush whooping knife.