As to your original question... Dimensions?  A lot depends on your venue and what you wish to achieve... I have known some high arch theaters that like to use 12 foot or better on their height.  Other smaller venues use only an eight foot height, which is enough to be representational… At my principal theater, our light batten is at 14 foot, so I typically build at 10 foot. (Although my most recent set went well beyond the batten to within a foot of our 21 foot ceiling..!) Anyway, once the height has been determined for YOUR standard flat, the next thing to decide is width.  If working with luan, the best and easiest is obliviously four feet as that is the width of the sheet.  If, however, you want to build a variety of flats to fit the vagaries of different set designs, the normal adjunct flats run in 1ft, 2ft and 3ft, all easily divisible from the standard sheet of luan.  Again, you will most likely want more of the standard four foot flats than the fill in flats, but that is something you will most likely decide by experience over time, and dependent on you most popular styles of set design and the layout of your space. (A little different for everyone).  With your standard stock you can do some mixing and matching as required.  Let’s say you decide on an 8 foot high standard and for a particular section of the wall or on a particular set you would like to go to 10 foot.  Often, you can just lay a 2 foot wide flat on its side across the top of your eight footer.  You will also need to decide on door and window flats.  They can be made up as a standard to be reused or they can be onetime units built to fill the space.  We often scratch build these mostly for stability sake.  Because stability is our goal, they are often framed with larger stock (2x4  etc) and that gets pretty heavy to haul around and keep together.  And we often want extremely different window styles or certain types or dimensions doors on a particular set.

With that being said, you have another decision to make before you can start building you flats do you want “Flat” flats or what some call “Hollywood” or “TV” flats. (I don’t know for a fact that the name comes from movie/TV set flats, but I assume so)  A “Flat” flat is build much like the muslin ones you remember.  Usually a frame of wood (I use 1x4’s) laid out to the dimension of your luan.  Gussets of wood (usually plywood or even other pieces of luan) or metal L braces are often used in the corners to help it retain its shape. If making an 8 footer, then obviously, the luan height will not need to be added too, if taller, than a brace will be needed at the joint height. Personally, I have started using the “Hollywood” flat.  It is a little harder to store, but it provides good connecting surfaces flat to flat as well as a good surface for connecting to the floor (or gives more stability to the flat on those occasions that when you can’t secure to the floor, like when I do my kid shows on the gym floor at the high school where I work)

The basic design difference here is that the frame is made like a box vs. a picture frame in the basic flat flat format. The luan is attached to the thin side of the frame board and can (to some degree) stand on its own.  If this was way too much information, sorry.. If you should have any questions feel free to ask..  Just remember I am only one builder and there are many variations to the concepts…

In our area, In the five theaters I work in three are hard flat users and two still use primarily soft... thats why I started the poll in the poll section to see what the broader world is up to..