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.
Every harness comes with 2 chest straps, besides the M size, which is with only one chest strap!
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)
L/XL size: fits all large breeds starting at a chest size of: 23.6 inch (60 cm), fitting up to a chest size of 45.3 inch (115 cm) and a lower neck circumference of 17 to 25 inch (43 to 63,5 cm)
XXL size: fits all giant breeds starting at a chest size of: 37.4 inch (95 cm), up to a chest size of 57 inch (145 cm) and a lower neck circumference of 21 to 33 inch (53 to 83,5 cm)