This harness sizing chart should help you measure your dog for the right harness.
Measure the shortest distance between the two points. Stay close to the neck.
Measure one half of the dog's neck from the top of the breast bone (sternum) to top front of shoulder blades (withers). Double this measurement to determine size. It is helpful to know dog's weight as well.
A handy test: measure as above but keep the tape in place at the withers to measure the other side from EXACTLY the same spot. Does the second measurement match the first? Not exactly? Start over until you repeatedly get the same measurement on both sides.