Measure a harness against its own size chart
Use a flexible tape and follow the manufacturer’s diagram for chest girth and any required neck measurement. Brands measure in different places, so a medium in one range need not match another. Write down the measurements and check the available adjustment range.
Check the fit on the dog
The harness should remain secure without pressing on the throat, rubbing behind the front legs or restricting comfortable movement. Check the maker’s fit instructions, watch the dog walk and sit, and examine the contact areas. Recheck growing puppies and dogs whose weight or coat changes.
Choose a bed for the sleeping position
Measure the dog when stretched out as well as curled up. Compare that space with the usable sleeping area, not just the outer dimensions of a raised-sided bed. Consider washability, access height and whether the dog prefers support at the edges or an open surface.
Before you order
Check the size chart, materials, cleaning instructions and returns policy. A breed example is only a rough illustration: two dogs of the same breed can need different sizes. Seek veterinary guidance if pain, restricted movement or a medical condition affects equipment choice.
Sources and further reading
American Kennel Club: choosing a dog harness — https://www.akc.org/expert-advice/gear/choosing-the-right-dog-harness/