I have a French Mastiff (weighing around 100lbs) and ordered the XL bed.  I have to say, she LOVES her new bed!  It is very roomy and my chihuahua is even able to sleep on it with her.  Because she is up in age, we were concerned about her having joint issues and wanted something that will last awhile, and most importantly support her weight without the mattress pad sinking into the floor.  We love the thickness of the mattress and all it has to offer... and at a reasonable price.  I highly recommend this to anyone looking for the most comfortable dog bed out there.