A coffee table or collection of coffee tables should be 1/2 to 2/3 of the length of your sofa. If you have a corner sofa it should be 1/2 to 2/3 of the horizontal seat. The height is best when it is the same height or a few cm less than the seat height of your sofa. This way the coffee table will not obstruct your view and remains functional. Side tables that sit at the side of the sofa should be within 5cm of the sofa arm height. Again this makes placing your coffee easier.