"3.1. A pilot’s circle shall be clearly marked. That circle shall have a minimum radius of three (3) feet but not larger than five (5) feet in radius."
So, 6' to 10' diameter is legit (for the mathematically challenged). There is no specific penalty for getting outside of the pilot's circle, but one can be DQ'd for flying in an un-safe manner.
I would add that some colors of paint may work better than white, depending on the current color of the grass. White works on green grass. On "summer tan" grass, black might be better. Since John has his own flying field, it might be good to concrete the center, or at least lay a 10' ring of bricks. Somebody (Tulsa?) bought a concrete septic tank lid to use, as I recall. You can also get concrete discs of various diameters for landscaping 'lawn steps'.
Steve