my board is about 16″ by 20″ and about 3/4ths of an inch thick. Basically, you want something large enough to cover the dome and have an overhang of 3 or so inches. You want it to be small enough not to have a mega over-hang or the weight distribution might be off (aka sliding). I have mine as thick as 3/4″ but you could use something thicker, but I wouldn’t go thicker than 2″ just as a personal preference (makes ceiling splits harder!) I’m afraid to go thinner because I rent, not own my house.