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You can quickly see the size of the mailboxes in use within your 365 tenant via the 365 Admin Center. Login as a global admin (or global reader/reports reader) and navigate to:
Reports | Usage
Under the Email activity heading select View More and then the Mailbox Usage tab heading
[Note: You can add additional columns to the view, including deleted item size and quota levels, via the button at the top of the list].
