Measure your dog and compare with the dimensions on the website. There are also some example breeds for reference. You might also be able to get some guidance from our dog bed size guide, as the small bed fits in the Fido Nook 24 and the Medium in the 36, but these will not be 100% accurate.
It's important to remember that puppies grow quickly, so make sure you get a size that will fit your dog as they grow.