but how much sail area would be my question. I only use the mizzen at anchor in light-ish air, to counter act current say, that has us beam to an annoying chop, or to change the sun angle, etc.

If in high winds, a boat swings wildly at anchor(some do even on a mooring), I don't think there will be much effect from some of the smaller riding sails I've seen.

On the other hand, I wouldn't be happy listening to my mizzen slatting in 20-25 knots of wind at anchor. My boat seems to be one on those that sits head to wind even on an anchor/minimal chain and nylon rode. I bet the mizzen mast and rigging helps combined with the low topsides.

Is there anything you can do to minimize windage forward of mid-ships? If you can get the wind's grab to go a bit aft, you might accomplish more than a riding sail.