Airstones don't actually add oxygen to the water. It's the surface aggitation of the water that adds the oxygen - the more aggitation, the more oxygen, so smb is right, larger bubbles make more aggitation, therefore more oxygen.
If your looking to find bigger bubbles, just take some knee hose and slip it over the bottom of a flower pot, than stick your stonejunder the pot andturn the pot up-side-down with the stone in it. You should see some big gurgles.
Self Experiance,