Bile duct size on ultrasound?

Hi----I had another ultrasound last week, "showing no gallstones", --sigh--but have GB symptoms. Anyway, the report showed that my bile duct is "4.2 mm" in size----if that is normal size (don't know what NORMAL SIZE is--anyone here know that info?)--and the scan "showed" no stones, is that a pretty safe bet that NOTHING could get stuck coming out in a flush? I am still scared to do it, but may come around in awhile---I did read somewhere on here that someone passed a 15 MM stone!!!!!!!!! Could that be possible? How large would a bile duct have to be to let THAT out? Sorry for all the dumb questions, but they are important to me! Thanks.