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.
S size: This harness fits at a chest size of: 40 cm to 73 cm and a lower chest circumference of 25 to 48 cm
M size: fits all medium sized breeds starting at a chest size of: 50 cm, fitting up to a chest size of 75 cm and a lower neck circumference of 35 to 58 cm
L/XL size: fits all large breeds starting at a chest size of: 60 cm, fitting up to a chest size of 115 cm and a lower neck circumference of 43 to 63,5 cm
XXL size: fits all giant breeds starting at a chest size of: 95 cm, up to a chest size of 145 cm and a lower neck circumference of 53 to 83,5 cm