Mailbox size reporting

Hi all - I'm a relatively new administrator and need to run a report
against my (5) 6.5 postoffices that only shows mailbox sizes for 90-days
worth of messages.

I can run a full report and get the total volume per PO (listed by
individual mailbox sizes) - I want to do the same thing but only show
volume based upon 90-days worth of mail.

Thanks in advance!

Steve