Consider these great tools to analyze the size of mailboxes, prior to migrating to Office 365 with Exchange Online. Although (as of Jan-2015) each Office 365 mailbox enjoys a quota of 50 GB, you don't want any nasty surprises! You can also advise users of the likely time it may take to export/import their mailboxes.
Exchange 2003 System Manager
Launch "Exchange System Manager" from the Programs menu and navigate to the "first administrative group" > "Servers" > {name of the server} > {name of storage group, such as "First storage group"} > {name of mailbox store} > "Mailboxes". From here you can tap the column headings to sort by size or total items.
Exchange 2007 onwards: Exchange Management Shell cmdlet
Launch PowerShell and type in this cmdlet:
Get-MailboxStatistics | Sort-Object TotalItemSize -Descending | ft DisplayName,@{label="TotalItemSize(KB)";expression={$_.TotalItemSize.Value.ToKB()}},ItemCount
The result (shown above) sorts mailbox usage by size and items.
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