If you're looking for a Unity tool, then the best one I can think of is Message Store Manager. You can find it here:
http://www.ciscounitytools.com/Applications/Unity/MSM/MSM.html
The help file will detail the info you can get from the tool such as the Subscriber Message Store Status Dump which generates a CSV file with data about each included subscriber’s mailbox. This data is extensive and detailed and is intended for analysis using a tool like Microsoft Excel or a database management tool.
Outside of that, you'd have to look into some Exchange-based tools which in a Unified Messaging setup probably won't be the most accurate (MSM may even be skewed a bit).
In general, if you use G.711 then a 1 minute message = 480k. The industry average for a message is typically said to be 30 to 90 seconds so most folks go with the easy 1 minute rule if they are trying to determine the impact on the message store prior to deployment. The other thing to consider that is not as easy to determine is the impact on Exchange transaction logs (i.e., message drop, message delete, etc - these types of transactions that can happens 100's or even 1000's of times a day depending on the size of deployment).
Hailey
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