The Weymouth should be a snug fit and you don’t need to see any of the mouthpiece. The lips should be allowed to spread naturally and brush up against the shank.
If you have a fixed cheek for a bridoon the fitting would be the same as above for the Weymouth and it is likely that both bits would be the same size.
If the bridoon is a loose ring there is normally a ¼” size difference between that and the fixed cheek Weymouth. This is because the general rule for a loose ring would be to have clearance either side of approximately one eighth of an inch from the corner of the lip up to the hole that the ring passes through.
A general guide to ensure your horse’s bridoon bit is at the correct height in the mouth would be to aim for ½ – 1 lip wrinkles at the corners of the mouth. Please bear in mind that mouth conformation varies and some horses are much shorter from the corner of the lip to the muzzle (‘short smile’). In this case it is often tempting to shorten the cheek pieces excessively thus causing the horse much discomfort and to appear to grin like a Cheshire cat.
When the curb chain is attached correctly it should restrict the shanks from rotating any further back than 45° in relation to the horse’s lips.