Dog Bed Ideas for Medium/ Big size dogs. we have struggled and struggled finding the right dog bed for our dogs and finally one day, I thought, why not buy a baby bed mattress. I went to our huge store- American Furniture warehouse and picked up a Posture Pedic Baby bed mattress for like $29 on clearance. It measures 48 inches long, 28 inches wide and 6 inches thick. this is gonna last forever, we just bought baby bed fitted sheets and then threw a comforter on top, our dog loves it, no sore spots on her shoulders anymore.