Even small dogs can be big barkers. If you want to put a stop to constant dog barking once and for all, check out bark control for small dogs.
If your dog is AT LEAST 12 POUNDS you probably don't need a special bark collar for small dogs. See Bark Control Collars.
DOGS UNDER 5 POUNDS: If your dog is 5 pounds or less even the smallest bark control collar would likely be too bulky for your little dog. We have heard great results from owners of really small barking dogs when they tried the PetSafe Ultrasonic Handheld Bark Control unit which does not require a collar. Check it out.