It is important to choose the right size of collar for your dog. If your dog is a puppy, you should choose a size that allows him room to grow. If your dog is very small, (under 20 pounds), leave only one finger's width between the collar and his neck.
The ideal dog collar fit should allow for one to three fingers to fit between your dog's neck and the collar, depending on the size of your dog.
If you have an average, medium-sized dog, go for a two-finger fit.
If your dog is very large, a three-finger fit may be better.
For more information, please read Choosing the Best Collar for Your Dog.