You might want to use some herbal remedies to help break up those larger stones. Gold Coin Grass tincture is supposed to help crush those stones while still in your gall bladder, so they come out in pieces instead of huge chunks. Note, however, that I've never tried it myself, so my data is hearsay. Your mileage may vary. Chanca piedra, I think it's called, is another one I've seen mentioned. Read through this forum and the archives for more info. Check out the testimonials forum, too.
Then again, my wife passed a "grape-sized (and -shaped)" stone last week. You mention that it's "5 cm" in size... Is it a 5 cm sphere, or a 2 cm x 5 cm egg-shaped thing? If it's the latter, then the flush should go just fine for you; pressure will tend to make it go "long-ways". If it's more sperical, then I'd be worried, too.