When choosing a small dog bark collar, there are two main options for you to consider: shock collar, or sound-emitting collar.
Shock collars have two metal prongs that stay in contact with the dog's neck. The prongs themselves do not hurt or bother the dog. There is a sensor that detects when the dog is barking, and then a battery in the collar produces a shock through those prongs. The shock startles the dog, but does not injure it.
The problem with using a shock collar on a small dog is two-fold. First, the box that contains the battery and the shock prongs are usually pretty bulky. They are designed to provide a strong enough shock to get the attention of larger breeds, so the device is usually too big for small dogs. Also, owners may be concerned that the shock strong enough to stop a large dog from barking may be a bit painful for a small dog.
The best anti-barking collar for small dogs are the kind that emit a high-pitched frequency. The profile of the device is smaller than the shock collar, and it is less likely to hurt a small dog.
So when you are trying to get a small dog to stop barking, get a small dog bark collar that uses sound, not shock, to deter the barking.
Dog Barking Control information from http://dogbarkingcontrol.net