If you already have a bit that fits your horse well, you can use it as a reference for choosing the correct size.
If not, measure your horse’s mouth width by placing a bit sizer or a soft cord/string in the mouth at the position where the bit will sit, mark the ends, then measure the distance. The bit width should be such that the rings rest just outside (or very slightly beyond) the lip corners—if the bit is too narrow, it will pinch; too wide, it will shift side to side. For loose ring bits, allow a little “play” (1cm on one side) so the rings can rotate freely without pinching the lips.
Width: 11.5 / 12.5 / 13.5 / 14.5 cm
Ring Diameter: 70 mm
Thickness: 14 mm