Dogs have sleeping patterns, just like people. Before purchasing a dog mat observe the way your dog sleeps. If Fido sleeps curled up in a ball, a round fleece nest bed would be your best bet. However, if he snoozes sprawled out, an extra large rectangular dog bed would suit him perfectly.  When  you are in doubt, it is always better to buy a bed one size bigger than your dog, that way he can stretch out and be completely supported and comfortable.  Most dog mats come in a variety of sizes to accommodate all breeds.