A harness works best when it fits securely without restricting normal movement.
Measure the chest girth
Use a soft measuring tape around the widest part of the chest, usually just behind the front legs.
Measure the neck where required
Follow the specific product size chart rather than assuming one brand matches another.
Check adjustability
A harness with multiple adjustment points can provide a more precise fit.
Perform a fit check
The harness should sit securely without rubbing the armpits or restricting the shoulders, and it should not be loose enough to escape from.
Recheck growing animals regularly.