Back Length:  Start from the base of the neck just above the shoulders, where a loose fitting collar would rest.  Measure straight across the back to the base of the tail.
Chest:  With your dog standing in a natural comfortable position, measure around the largest part of the chest directly behind the front legs.
Neck:  Base of neck circumference.

One piece design. Pull over your dog's head, pull the front feet through the front sleeves, then pull to the back. Bottom and rear are open, so your dog can still use the restroom without removing his PJ. Cuffs may be rolled up to make them shorter.