So many people ask me what to look for in a bosal that I decided it was easier to have some made than explain it. We begin with a 5/8" width, which I've found is the most versatile, then you'll notice that the nose piece is a little shorter than you see on most and that is because I like the balance I get this way.
The cores are made from rawhide so that they'll bend if they need to, but always spring right back when released. Another thing that's critical is that the rawhide strings have been beveled before being braided into the piece, so that there aren't any hard edges to rub on the horse.