That value I "guess" means how far your toon can see. For example, if you put search radius 20, then your toon will only detect "mobs" or "nodes" whatever you're farming, within 20 yards radius of your toon. This is just my understanding, I could be wrong.
However, from my experience, having the radius too small will restrict its movements to certain angles. Therefore, it makes it a little more obvious. I suggest keeping it high value between 100-200 so it would look a little bit humanized.