Generally, but not always, when a bed manufacturer lists a bed size they calculate it based upon the dimensions when they cut the fabric. Once it is sewn and filled the size is approximately 2-3 inches smaller each way. So when the bed carries a label reading, for example, 40x30 it may in reality measure only 37x27.
That's why we recommend measuring your pet accurately and then adding 5-7 inches just to make sure your pet can rest comfortably. When in doubt, try to error on the larger size rather than the smaller size. Your pet wont mind stretching out once in a while but sure wont like being cramped.