I've had several CT estimated stones at 5 by something bigger, which turned out to be 4 by something big when I passed them. It took a little longer than my usual 2-3 weeks for the big ones but the urologist said he was willing to wait up to 8 weeks.
The size estimates on X-rays and CTs are guesses and the radiologists that read the CTs don't usually hear back from the patient with the actual size, so they can't improve their guessing skill very easily.
I would think that a stone that size will have to be removed by your Urologist. My Urologist has told me that it's possible to pass a stone up to 5mm but that a stone 5mm and up is usually removed by surgery. A 5mm stone is one the border line and it would depend on the person and circumstances as to whether it can/will pass on its own.