Wow. That'd require a lot of work! Most of my databases (individually, not collectively) would fit on my 8 gig flash drive, my biggest is our ERP system which is 24 gig. So anywhere from a few dozen meg to 24 gig for my place, but any answers would be wildly variable. I don't really see the database size as significant, though. You'd also need to consider how many servers, how many instances (not the same thing), the organization size, what they're being used for, and are you including the system databases, which are also databases, just not directly production.