Almost every harness that comes back does so for the same reason: it was bought on the dog's weight, or on the breed printed on the size chart. Neither is a measurement. A deep-chested spaniel and a barrel-chested staffie can weigh the same and need different harnesses.
Take two measurements
Girth is the one that matters. Run a soft tape around the deepest part of the chest, just behind the front legs, and keep it snug rather than tight. Read it in inches.
Neck is the second. Measure at the base of the neck, where a collar naturally sits, not up near the ears.
Size on the girth
Every size chart on this site is built on the chest measurement, because that is the strap carrying the load. The neck measurement is there to tell you whether the top strap will sit comfortably once the chest is right.
Between two sizes, go up
A four-point harness takes in about an inch and a half at each strap, so the larger size adjusts down to fit. The smaller one does not adjust up. If the tape lands on the boundary, take the larger size.
The two-finger check
Once it is on and adjusted, you should be able to slide two flat fingers under any strap and no more. Less than that and it will rub behind the legs; more and a determined dog can back out of it.
If you are between sizes and unsure, send us both numbers and we will tell you which one to take.