The limitations of our own minds prevent us from being able to explain God well. A circle is a good image because it is continuous. But it fails to encompass all, so at the same time it is not a good image. The surface of a sphere makes a good analogy, too, since any point could be the center and you can travel infinately in any direction. But it still leaves an untouched existence inside and outside of the sphere. To experience even a brief moment of infinate time and space, we must let go of the edge and hold on to nothing. Or so I have been taught, since I have not really acomplished it yet.
I take it as meaning that since there is no circumference, then there are no boundaries. God is love without boundaries to stop it. About how to know it is a circle, this is the example I think of: We know that the world is essentially round (not exactly, I realize, but for this purpose close enough) because we know that if we start walking at one point we can eventually come back to the same point. Same thing with God. You can try to run as far away from Him as you can, but still find that you run right into Him, because He is in all places because of this circumference.