With daily purging, that scenario will result in a very stable database size once all of the purging has kicked in. How about you set your system up, let it run for a month, look at the DB size then, and then multiply by 12?
I understand that it would be nice to have something automated - believe me i do. But there are a great number of things that are planned for this system. This one honestly has a strong case for remaining mostly manual.