...or, even easier (and more permanent!!) you can buy a BitFit Bit Measure for $30 from my website. In stock now, ready to post to you!
Every horse is different, and a large number of people are using bits that are too big! The shops selling Pony, Cob and Horse bits are not helping, as you might get a thoroughbred in a 4.75", a WB in a 6", a pony in a 5.25"- the size of the horse does not dictate his bit size.