The only measurement you need to pick the proper size is the chest girth, measured all around your dogs chest starting at the lowest point of the chest right behind the front legs, up to the back of your dog.
Size chart:
S size: This harness fits at a chest size of: 40 cm (15.7 inch) to 73 cm (28.7 inch) and a lower chest circumference of 25 (9.8 inch) to 48 cm (18.9 inch)
M size: fits all medium sized breeds starting at a chest size of: 19.7 inch (50 cm), fitting up to a chest size of 29.5 inch (75 cm) and a lower neck circumference of 13.8 to 22.8 inch (35 to 58 cm)
The SMALL size comes with 2 chest straps for a better adjustment according to the dogs body shape.