How do you find the right bridle/ fit it ti the horse?

You can measure the horses head from the corner of the mouth to the ear and take those measurements to the store. Or if it's a small horse like a quarter horse you'll probably need an adult small bridle which is usually how they are sized. I never actually measured my horses I just knew what sizes they were. The bit is probably a little more special fitting as it does not adjust. Tell the clerk at the store what kind of horse including age and height and they will be able to help you. Eventually you'll just know.