Most medium sized horses take a 5", though some need smaller or larger. Your best bet is to measure whatever bit she currently uses. If this is a new horse, then get a nice straight stick or thick bit of twine. Mark or knot one end. Slip it in her mouth so the mark/knot is at one side of her mouth. Make sure it's straight in her mouth, and taught if you're using twine. Mark the other side with a felt tip pen. Measure your twine/stick. Get a bit with the closest measurement. If she's between sizes, go up one size.