I think a 14 inch would be the longest I'd go, but it also depends on how you wear your hair. If you do a fish tail braid, I think you could get away with longer so that the braid comes from left to right (I'm assuming down your shoulder)? That way the braid will be showing. If you decide to do something else, like romance curls, then you may want to go a little long just so that they stop past your shoulders.
I think you should go a little longer just to be safe; you can always get it cut/trimmed later.