There a lot of folks here who have passed stones as large as an inch. I have one over 4cm (almost 2") and I'm prepping for the flush myself. I'm no Dr (thank God) or an expert, but the Epsom Salts should dilate the bile duct adequately to let it pass on through. And the oil lubes it as well.
Like you though, I'ld welcome some comments/reassurence from someone who's passed a gb stone that big. Aug 2 is the BIG day.