As a proud mom to a 6 pound maltese named Hemingway and a retired breeder of specialty toy breeds, finding the right collar for a pet has been a matter of finding something small enough without sacrificing comfort and quality.
For owners of small dogs I recommend purchasing a small harness verses a collar. I would also avoid buying online and instead taking your tiny friend to the petstore for a fitting. Tiny necks are fragile and if you intend to lead your dog on a leash, a harness is the safest way to go.
If your dog is long or fine haired like a maltese, teddy bear, or yorkie, try sticking with cloth fabrics verses metals or plastics as these cause mat's and tangles. The size rule of thumb for tiny dogs is two fingers fitting between the collar and the dog's neck.