A Fitting Collar

To find a collar that correctly fits your dog, take a measurement of her neck. Use a soft tape measure or string that you can place against a ruler. Take that measurement and purchase a collar that is in the neck size range for your dog.
To know if you've bought the right size the collar, check if you are able to fit up to two fingers underneath your dog's collar. However, the collar should not be so loose as to slip over your dog's head.