Apologies for this basic of question but is there a simply method to report on the compressed mail archive size for the entire site?
If possible I would rather not start running SQL queries on the backend as I was informed that this is potentially dangerous to the running of the application.
SQL query might be easiest. Usage.asp pulls information about archive points from the table named "ArchivePoint" in the database to determine the Archived Item Size. This value is the total size of items in the corresponding archive/vault store and does not represent compression or single instanced data. i.e. - this is the file size prior to archiving.
In the console if you go to Enterprise Storage Management and select "Run operation reports" you can then select the "Single Instance Storge Reduction Summary" and get the data you are lookiing for.