There are 3 measure points to take when you measure your pet:
Neck (A): Measure around the back of the neck, where is join to the body.
Back (B) : Measure from base of neck to base of tail.
Chest (C) : Measure around the chest at the largest part behind front legs.
Of all the 3 measure points, Chest is being the most important of all, because you want to make sure your dog can fit into it.
The least important one is the Back measure, because it is not necessary to have the cloth to cover his/her full body. In fact, a lot of people prefer the cloth to tuck up short so that it will be less messy when he/she does their "business".