Dog Beds.
The size shape and age of your dog will determine or atleast limit the dog bed you buy for him.
A verry small breed dog can use most dog beds but you should buy the one that fits him best seeing as small breed dogs tend to live longer on average it will be worth the investment.
Medium size dogs should be fit to a bed atleast equal to their body size.
Large breed dogs will probably be the hardest to shop for and may need special attention to size and breed requirements due to hip dislasia and the likes.
Older dogs may require asistance from a vet to determine the correct firmness to keep from harming the spine.