Jmuraite...
I've got two possible solutions for you:
1. There is a command called "Rough Stock" that computes the minimum stock size based on your partbody, and it creates a volume of faces for your reference. I recall it requires a NC license, but I forget which one. You can find some more information about it by doing a search in the help files.
2. When you do an Inertia analysis, CATIA draws a 3D box around the part. If you keep the measures, you will find 3 BBL values at the very bottom of the tree. These are the Boundary Box Lengths in the x, y and z directions. (I think this is what you're looking for?) You may have to play around with an axis to change the orientation.
I don't recall if either of these solutions provide the update capability you want. I'll let you check them out and report back. But both are very quick and easy to use.