The simplest way is to use a selection node and let someone select the appropriate elements, then use their geometry to find the surface area to subtract.
The other way, that I can think of, is to filter through the windows and doors in the project, and find the ones that are in that room. One could probably do this via the API using a FilteredElementCollector, but in lieu of that, try checking out this thread:
You could also use that clockwork node to get the walls associated with the room, and get the faces of the walls. From there maybe you could use the edge loops or something and figure out how to trim down the faces to just the sections you need.
Those are what come to mind for me. Either way it sounds like you got your work cut out for you.