I see. A flat window that you can build on. Well, at the moment there's always the option of taking print plates, which you can build on, and building some bricks to hold it up, then building a ceiling print plate on top of those bricks, and then a normal print plate on top of that, to avoid any brick-textures showing up. (Or if you only need it to look good from one side, you can just use one print plate.) With that method, you should be able to build "flat" windows of any size.