This was discussed at length (and got off-topic, as there are several ways to determine zone area):
http://archicad-talk.graphisoft.com/viewtopic.php?t=3304
This would defintely be a big help, especially since zones work well as bubble-diagram space planning tools. I wish I could create a zone and get real-time feedback on the area, so as to create a specific room area template with arbitrary dimensions (i.e. I need at 520 sf room that's as wide as this adjacent office). There's a great routine for this in, of all things, the free NCARB Architectural Registration Exam sample test software. When you draw room in the Space Planning vignette, it gives automatic feedback for a quick layout:
http://www.ncarb.org/are/tutorial2.html
Chuck Kottka
Orcutt Winslow
Phoenix, Arizona, USA
ArchiCAD 25 (since 4.5)
Macbook Pro 15" Touchbar OSX 10.15 Core i7 2.9GHz/16GB RAM/Radeon Pro560 4GB