Well the current rule for a 200 amp panel using an artificial electrode is #6. The fact that the ground is connected to the water pipe could require a ground wire upgrade under T-17. I would dig in a plate with #6 and call it done or add 2 - 3 meter rods. Use the existing ground wire to bond the internal water piping as long as it is at least #6 copper.
In any house where they used a ground rod more than a couple of years ago I would think it reasonable for an inspector to get you to prove the electrodes are not rusty spikes.
