Neck
Using a tape measure or a length of string (measure with a ruler), measure right around your dog's neck where a collar would typically sit. A comfortable fit will allow two fingers (flat) to be placed between the collar and your dog's neck.
Chest
Using a tape measure or a length of string (measure with a ruler), measure right around your dog, starting at its shoulder blades, down to the deepest point of its chest, and back up again. This measurement is usually taken from just behind your dog's front legs.
Back
Using a tape measure or a length of string (measure with a ruler), measure from your dog's shoulder blades down to the base of its tail.