A lot like us, each dog varies in size and physique. One person's idea of small or large can be very different from another. Even within breeds there can be vast differences, depending on stature and the thickness of coat. Seeing each dog is unique, we must accomidate for these variations, To accurately choose the right sized garment you'll need to obtain some measurements.
Use a soft, flexible tape measurer.
You can also use string or ribbon, and obtain the results with a straight measure afterwards
Length: Make sure your dog is standing. Measure from the base of the neck along the dog's backbone to the base of tail.
Neck: Measure the circumference where the collar would naturally sit, at the widest point, around the base of the neck / top of shoulders.
Girth: Measure around the broadest part of the chest or rib-cage located behind the dogs front legs