The Alinor Ballroom floor is the largest flat piece currently available, well as far as I know, but I have invested ton of time into fiddling with furnishings. The Ballroom has couple of issues to keep in mind, the first is that it is very very very thick, and thus is really difficult to use as a middle floor in interior spaces. It also has annoyingly rough edges that further limit it's use in said role. However... As long as you can sink it at least partially into the ground, those things will not be much of an issue and it does provide a large flat surface.
Well, almost flat, since it too has that annoying trademark ZOS "if it's busted and broken, it's more realistic since people don't give a *** about the condition of their homes" feature. But oddly enough, the bottom of the Ballroom is pretty much intact. So if you flip it over, you will get a nice smooth surface on which to build stuff.