Please notice "neck girth" and "neck base" are different - the attached photo will show you how to measure. Make sure you measure "neck base", so that the harness won't be too small.
I always add 1 inch by the neck base measurements. EX: If your dog's neck base is 14", I will make 15" - so that your dog can wear the harness comfortably. So don't measure your dog "too loosely", otherwise the harness will be too big.