I have a 13 year old, 27 lb. Westie, named Scottie, who is also 27" long lying on his side with chin extended (not including his tail)!
I first found one of these beds at Petco about 5-6 years ago, and you could tell by the feel of the fabric and cushion that it was much better quality than all the other "average" $30-45 ones they had for sale. I had gotten a large (brown) size of this Dog Gone Smart Bed brand, and in the store it seemed a bit smaller than I thought Scottie deserved, but they said I could return it if too small. I didn't need to. Scottie fits in it just fine, and loves it. He either sleeps curled with head and body inside the donut, or lays his head on a side edge acting as a pillow to keep an eye out for squirrels in the back yard. After just washing it the 5th or 6th time, it looks as good as new. Westies hardly shed, so if your dog does, I would recommend a Khaki or Olive color
I wanted to quit replacing the second "average brand" I keep buying to save money for another room location. The latest casualty is a Woolrich brand oval bed with 6" sides, that looks horrible, with worn, bare spots after about a year. I finally remembered that "penny wise; pound foolish" addage, and am buying a second one after I finish this review.
I checked the inside label of the long lasting bed, to find it was this "DogGoneSmartBed" brand, and went to their website (add a dot com to their name) to see what was available. Their listing of the large size (like this Amaazon product listing) category says 35" & is supposedly for dogs up to 50 lbs--No Way!!! However, below their main size categories (on the same page), the manufacturer gives additional specific dimensions of inside/outside measurements which are not correlated with above categories. For example, the large is not 35" x 35" outside diameter. I'm sure this is why people are being misled about ordering the correct size. I don't believe that their large size is going to work for a dog much larger than mine.
Scottie (27 lb Westie) fits comfortably, but that is with him in curl--and not much room to spare if he was larger. I just measured my old large size and posted a photo I took at the top of this product photo listing. The bed ends up being more of a circle than an oval, and the outside diameter of my donut is 31", inside diameter 19-20", wall thickness about 6". The inside rectangular cushion insert is 25x19". There are cleaning instructions for the outside liner (which unzips to remove self-contained padding roll) & cushion insert. Basically, you wash on gentle with small amount of detergent, and tumble dry on high heat.
I am ordering another large size today, because the quality, durability, water resistance is really worth paying more starting out. It is well worth the money--based upon the wear so far, I am sure this will last 15-20 years. I would say get the medium if your dog is up to about 18 lbs. Large if 18-30 lbs. Extra Large if 30-55 lbs. I would expect larger than 55 lbs will be too large for this kind of bed. Maybe 60 lbs...but not sure.