Please use a flexible tailor's style tape measure. I prefer you use inches.
Measurement 1. Chest at the largest circumference behind the front legs Measurement 2. Chest at mid-back Measurement 3. Loin, the narrow circumference at the stomach
Measurement 4. Have your dog stand with his/her weight centered on the front leg closest to you. The front leg should be perpendicular to the level ground: your dog should not have the leg closest to you stepping forward or backward. Start the tape centered on the protruding chest bone and run it under the dog, stopping just behind the front leg, just past the armpit. This measurement is typically 7" for a 50-55lb dog.
Measurement 5.From just behind your dog's front legs to where the ribs start to turn upwards at the sternum. This is about 6" for a 50lb dog.