Mike C,
Usually,the longer the blade on a knife that a "woodsman" wears indicates the more of a green horn he is.
Three to four inches is pretty much the standard and can get most stuff taken care of.

Clearing of brush,cutting up saplings etc is much better handled by and axe,saw or Woodmans PAL...at least in my opinion.

As for self defense knife.............my advise is distance and plenty of it. Trust me on this,in a knife fight,no one walks away (if they are able to) unscathed.

Wayne