Garment measurements with reference to Ladies size S are: shoulder width of 37 cm, chest circumference of 88 cm, sleeve length of 57 cm, and torso length from shoulder to hem of 61 cm. (dresshead.com)
Not everyone who was average height (5 feet 9 inches) had the same chest circumference or head size. (gse.harvard.edu)
The FCI uses chest circumference in its standards for this type. (breedingbusiness.com)