View Microsoft Exchange 2007 Mailbox Sizes
This is a short tutorial for those who want a quick and easy way to view their mailbox sizes in Microsoft Exchange 2007.
Step 1
Launch the Exchange Management Shell.

Step 2
Copy and paste the following into the shell window.
Get-MailboxStatistics | Sort-Object TotalItemSize -Descending | ft DisplayName,@{label=”TotalItemSize(MB)”;expression={$_.TotalItemSize.Value.ToMB()}},ItemCount

Step 3
The mailboxes will then be displayed in three columns, mailbox display name, mailbox size, and item count.

Check out this link for information on exporting archive sizes via Powershell to CSV from Office 365 hosted archive services.
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