The most important measurement is the width of the bed. What you want to do is have your dog stand up straight, and measure from the bottom of the paw to the top of the hip or shoulder.
This is the amount of room that your dog needs to be in a full 100% stretch position.
Compare that measurement to the width of our sizes. You'll want an extra 3-4 inches.
For example, if your dog measures 25", you'll do great with the Large, which is 30" wide. But if your dog is 28", you'll want to bump up to the 36" wide XL.
Note: the foam composition is the same between the Large, XL, and Giant sizes. All 3 sizes can easily support a 300 lb dog if they have to.
For 100% accuracy, please watch video above and take a quick measurement. However, in most cases, the following examples will work for the average sized dog for the breeds listed below.
The Giant is also recommended for two large dogs that share a bed.