I just saw this short video made by Autodesk about CEA and the ability to have parameters driving a consistent floor square footage:
http://www.youtube.com/watch?v=KFLDzbuOZVU&feature=player_embedded#at=82 (start @ 1:20)
I'm really interested in how it's done. I've googled it but can't find a guide for it so I thought I'd post this here.
Thanks!
You can download the Revit file shown in this video from this location: http://revit.downloads.autodesk.com/download/RMEContent/CEA_Financial_District.rvt. Note that there are design options in this file. Access them via the ribbon or the bottom of the view frame. The 3 facetted design options all have the same gross floor area. You can also open up the family to see how it is constructed. Please let us know if you have more questions.