The standard size for a stud on a Gibson humbucker is .187″ diameter by 1/2″ long. The normal stud polarity is North on a standard humbucker. You would need to keep the magnets as close to length as the steel studs. When you insert the magnet rods, you need to keep the magnetic polarity “North” facing the strings. The steel stud pole pieces are plated to keep from rusting and increases the diameter that keeps the stud firmly held into the plastic molded bobbin. Trying to grind or cut a magnet already magnetized for proper length can cause several problems. Grinding or cutting magnets will cause fine bits of magnetic dust to become airborne that are harmful if inhaled. Grinding can cause the magnet to get overheated and become degaussed or weaker. It would have to be magnetized to bring it back to its proper strength.