I have personally passed stones larger than 1.2 cm in diameter. Some were "grape-sized". My wife has also passed stones in this size range.
I have passed them both with and without flushes. Without flushes hurt a lot, as I'm sure you've gathered by now. With flushes, the most I felt was tingling while the stones were being pushed out. It was more of a tickle, and certainly nothing like the mind-numbing pain of a full blown gall bladder attack.
As to whether or not the herbs will work and the flush will work... You only get out of it what you put into it. These things have worked for many people. Chinese herbal medicine is pretty amazing. My only advice would be to follow the herbalist's instructions to the letter. Chances are, (s)he's suggested taking lots of the herbs you've been given. With herbs, more is *usually* (but not always...) better. So don't skimp unless you've been advised by your herbalist to skimp.