When measuring your dog for its new bed, you will get the most accurate measurements if you have your dog lie down. Measure from the tip of his nose to his rear or back legs. Then add about 8"-10" to the dog's length. When you are not sure of the size to order, it is always better to get a larger size.
For the dog that sleeps curled up, add 10"-12" to the measurement that is the longest distance while they are curled up.
Measurements for the different sizes of the beds are on the individual product's page.