Actually i think Scanner001 could help with this issue
all you need to do is run a basic SQL statement that does a SUM of archivedItemSize where the ArchiveDate is greater than x amount of days or IdDateTime is greater than a certain amount of days
so for instance if you wanted to expire anything before 12/12/2008 using the ArchivedDate it'd be like
USE VaultStoreDB
SELECT COUNT(IdTransaction) AS NumberOfItems, SUM(ArchivedItemSize) AS TotalSize
FROM Saveset
WHERE ArchivedDate LTE '12/12/2008'
or in this instance if you are using Sent Date (Modified Date in storage expiry)
USE VaultStoreDB
SELECT COUNT(IdTransaction) AS NumberOfItems, SUM(ArchivedItemSize) AS TotalSize
FROM Saveset
WHERE IdDateTime LTE '12/12/2008'
Column names probably need to be changed and im not entirely sure if DateTime fields like LTE/GTE etc