Im guessing you want to do this for all mailboxes, and not just one.
The only way i can think of doing this natively is a little bit risky. I naively assumed that this information would be in the report generated by running archiving in report mode. I checked, and to my surprise I was wrong -it's about the only count which is missing. However, there is the information on the average shortcut size, and the number of 'old' shortcuts. Old shortcuts are those that are past the expiry date and would be deleted by shortcut deletion. Therefore the plan entails making all shortcuts 'old' and then multiplying by the average shortcut size to get your answer. Hopefully you can see the danger with that (leave the policy in place and lose all shortcuts!)
At your own risk:
1) Make sure you are in read-only mode and just generally miles away from an archiving run. Back up exchange.
2) Set the exchange mailbox policy so that shortcuts are set to expire one day after creation
3) Synch the policy to the mailboxes
4) do a 'run now' on the mailbox archiving task, choosing report only, and making sure you report on all items. Choose the relevant mailboxes, or run against all.
5) Having obtained your report, undo the changes you made above: Change the shortcut expiry policy back to what it was before
6) Synch the policy to the mailboxes again
After that take the output report and import it into excel, where you can do the maths you need to work out your shortcut size. Remember though that this will be different from the actual size needed by the edb file (not all attributes report size, and there is whitespace to contend with) , and also different from the size you will see if in cached mode.
Alternatively, you could just have a go creating a powershell script or VBscript etc etc to recurse through all the mailboxes looking for specific message classes and then totalling up the values. An example which sort of does this, but is designed specifically for deleting a certain message class can be found here:
http://blogs.technet.com/b/manjubn/archive/2012/03/31/exchange-2010-search-and-delete-items-by-messa...
just remember to cut out the delete and add some counting in ;)
Regards,
Jeff
