Measuring for your dog for a Trig Point harness:

• Ensure your dog is standing still
• Measure using a fabric tape measure or string
• Measure the chest immediately behind the forelegs and immediately in front. Both sizes should fit within the adjustable range. 

Note: If your dog's measurements overlap two sizes, then unless you expect your dog to continue growing, or if your dog is large set, the smaller of the two sizes is probably the better choice.