Each Sweater is unique and one of a kind. They are not standard sizes. The stated sizes are generalization. Consider that some sweaters are tapered at the bottom and some aren’t, and also the arm widths vary. The chest size of the sweater should be 2-4" larger than the chest size of the person wearing it. The back measurement is from the base of the neck to the bottom of the sweater. The arm is measured from the centre of the back to the end of the sleeve.
Click to read about Cowichan sweaters.