As a general rule, when your horse is outfitted in his new bit, you should see two wrinkles in the corner of his mouth. However, this rule doesn't apply to all horses. Your horse's bit fit may be affected by his sensitivity, lip conformation, and even skin type (if his skin is tough, it may not wrinkle easily). If your horse has short lips, one wrinkle around the side of the bit may suffice.

Tongue Size:
You'll also need to check your horse's tongue size. If your horse's tongue is large, you'll see it between his teeth if you lift his lips while his teeth are closed. Choose a bit with extra tongue room in the form of a groove or hinge. A thinner mouthpiece may also help.

Bars & Palate:
Next, have your vet evaluate your horse's bars and palate. You'll want to make sure that the bit you choose doesn't take up more than half the space between his hard palate and bars.

Measure The Mouth:
Ready to measure your horse for the right bit? Using a tape measure or string, measure your horse's mouth from corner to corner, going through his mouth. Order a bit that is 1/2 inch larger than what you measured. He'll have enough room to avoid the bit pinching the sides of his mouth.