Stones much much larger than 15mm can be passed painlessly. Epsom Salts dilate the liver ducts. To ease anxiety you may want to try softening stones first either by apples , apple juice, lemons, malic acid, herbs such as chanca piedra and gold coin grass (there are many stone dissolvers, you get the point). Even if a stone gets stuck which is the very worst case senario you can do another flush right away to push it through. I personally have never had a stuck stone, a stuck stone would tend to be an abnormally large one.