Plenty of water. Increase in water intake to have a urine output of more than 2.5 liter is the only proven way of reducing risk of kidney stones. It is less likely for a established stone to become smaller over time, however modification in diet and risk factors can be helpful depending on the composition of stone.
None. No particular food or drink will decrease stone size. Some stones (e.g., uric acid) can be reduced or dissolved by alkalinizing the urine with medication. If you have kidney stones, you should be tested to determine the cause (s) and to see what possible interventiosn may be required (e.g., increased fluid intake, medication, etc).