put a string in their mouth where the bit would go & then that's your bit size
Also - my 13hh pony, my 15hh WB and 16hh petite TB have a 5.5" so height is a hard thing to go by.
Also take into consideration the size of your horses tongue, if it's thicker, usually a thinner bit is more comfortable and vise versa.
why not put things in the horses mouth? (Just curious)
so if my mare is 15.3hh, do you think she would need a slightly smaller size?
Sorry, hit enter before I finished typing. If you can borrow different bits that you would ride in from someone, try them and see which ones work. If you can’t do that, most horses take a 5-5.75” bit size. My guys are 16.3 and 17.3 and they both take a 5.75”. Hope that helps.
Do not put anything in your horses mouth to measure what size bit fits!!
As the bit is in horses mouth you should be able to put your finger between the ring of the bit and the skin so it's not pinching the horse (both sides)
Get a stick, measuring tape or something else and put it in your horses mouth where the bit sits and take a measurement of that and voila.