ya shoulder width is a good way to measure. i run either 25-26 or 28"(diff bars) on my two-six , 25.5" on my 20" and about 28" on my MTB.
26-27" is shoulder width for me. i can run a little less or a little more and still be comfortable. i prefer wider bars usually for the crazy amount of leverage you have, but running them around 26" or so is very comfortable. on my 20, i like to run shorter grips to make the bars feel a bit wider since i don't like gripping the end of the bars.