I'd think the width would depend on if you intend to mount any controls on it and how high the scenery is, among other things.
The scenery should determine the profile af the fascia, whether you cut the fascia before the scenery is in place or not.
Of course, if you cut the fascia first, then you have pretty much locked yourself into what the scenery is. so I would do the scenery first, then cut the fascia to fit.