You have to look at many important things when buying a king or any other size mattress. For instance, accessibility is essential! King size mattress will be perfect for a room with a minimum area of 12 feet by 9 feet, 10 inches. If you have a small room, a king-size mattress will cover most of the space. As a rule of thumb, it is best to leave 30 inches of space around your bed for easy movement.