Neck
Measure your dog around his/her neck as you would for a collar. Allow some extra room for a comfortable fit. 
Chest 
Measure dog's chest girth at its widest point, all the way around, which is typically right behind the front legs. If the dog has a large girth, going up a size is recommended. 
Length 
Measure from the base of your dog's neck to the base of the tail.