My dog weighs around 40lbs, and is similar in size to a heeler. After reading the reviews and size guide, I almost bought a large, but instead decided to go for a medium, because she likes to curl up in a tight little ball. Right now, she's curled up in a ball in her new bed and there is PLENTY of room around her. In fact, I might have to get a small to take to the in-laws' place to replace the worn out cat bed that she loves to squeeze into.
The material is beautiful, and the bed is very cushy. Wish they made a human-sized one!