My dog weighs but 35 pounds on her heaviest day but we bought her the XL. She LOVES it - it's like a puppy king sized bed. It's still going strong after almost 6 years and daily use. We may buy another soon just because ours is kind of dingy now, but the quality of the bed has help up extremely well.
This is my second mink grey napper bed. The original is now being used for nighttime sleeping in our front room and the new one is for daytime naps and hanging out in the family room . Both labs love them and take turns. Super soft and supportive