if you are going to order a polished horn then 14 inches should be plenty. However, if you are odering an unpolished horn then I would go at least 16 inches if not a bit larger. Unpolished horns are sent without the butt end having been cut. Sometimes you have to cut off two or three inches of unuseable horn on the butt end just to get into good horn.
Most of the unpolished horns I work with start out in the 18-24 inch long range. You can always make a shorter horn out of a long horn, but trying to make a longer horn out of a short horn is a lot more work.
i'm not sure if i'm going with a natural finish or polished yet....most likely it will be a plain horn fer my first one fer hunting....right now i'm useing an old chalk line plastic bottle :v .............bob