I don't think they are six feet wide -- they might be five feet, at most. Look at the height of the kid in red -- if he's six feet tall, then from his feet to his waist is no more than three feet. The panel isn't too much wider than that. In any case, you can get aluminum composite panels painted bright white on one side, and often used for signs, called Dibond, in sheets of up to 60x96 inches, of 5x8 feet. It's quite light and durable and stiff, and not too expensive. If you need bigger panels than that, you could always make fabric panels -- I think that might be a fabric panel on the aluminum frame in the foreground. These, you could make to almost any size and durability you wish. If you want to do it cheaply, you can buy polyester canvas in widths up to 40 feet (!) and almost any length, used in theater productions, mount them on frames, and paint them with acrylic gesso. That will give you a fairly white, seamless and reasonably durable surface. You can also buy stock sheets of white mylar plastic in rolls of up to 60 inches wide by 100 inches long, and mount them on frames.