Title: BITTING the horse
Subtitle: How do I measure for a bit?
Case: A young gelding and an amateur adult rider/owner.
Question: How do I measure width? Use a bit I have then slide it tight on one side and use two fingers to get the measurement?
There are two important factors to bit measurement:
1. The length: the measurement between the two bit rings. This is best measured with a dowel. Put it gently into the horse's mouth and let it rest where the bit will go. Using a permanent marker, mark each side of the dowel on either side of the lips where the lips rest comfortably with no compression (it may take a few minutes for the horse to relax once the dowel is in his mouth.) Use a ruler to measure between the marks OR, better yet, take the piece of dowel to the tack shop with you. This is your guide. Place any bit you think is right over the dowel. If the bit is a D-type ring with no places to pinch the lips, less than a quarter inch on either side of the lips (marks) is fine. Loose rings require that infamous "finger" though no one knows what a finger width is. A good rule is a quarter inch on either side plus a little for loose rings. If the bit is too long it will slide around in his mouth and hurt him. In all cases, you can't trust the labels on bits! Don't assume that a 5 inch bit really is 5 inches from the inside of one bit ring to the inside of the opposite bit ring. Trust the stick.
2. The width or thickness: the diameter of the mouth part(s) of the bit. This is tricky. Too fat a bit and the horse can't close his mouth comfortably on it; too thin could hurt, especially in unskilled hands that are not yet quiet and soft. Evaluating the shape of the horse's mouth (palate and tongue) is where an equine veterinarian or dentist comes in handy. You can figure it our using your thumbs and fingers to sort of feel out the amount of space between the bars vertically, along the bars horizontally between the teeth and between the tongue and the upper palate.
Unless you are made of money or you want a considerable bit collection (in which case you can be the person in the barn with bits to lend), once you have an idea of what might work it is a good idea to borrow bits and put them into his mouth to get a sense of what he's comfortable with. Slightly thinner is better than too fat. Milder is better than stronger.
Just because a bit is popular and everyone in the barn or the show arena is using one, doesn't mean it is the right bit for your horse. Bitting is individual to each and every horse. It may take some time to find the right bit. Give each trial bit a fair test by using it for several weeks. This gives the horse a chance to get used to the new feel and then to explore the feel. Always use the mildest bit possible for the comfort of the horse.