The radius determines how large or small the circle is that is drawn around the geopoint. There is not a maximum limit for the size of the radius, however what I have found is that the entry or exit off a fence requires triggering to happen, so with larger (10 km) maybe you're not triggering the entry or exit.
You mention that 10 meters works, have you tried 100? 1000? I would test with increasing size and make sure you're logging your information as you test so you can determine if you are actually triggering the fence.