You might need to find out the sizes of all your users online archive mailbox in Exchange 2010 or 2013.
To do this you can run this command in the Exchange Management Shell. It will return the top 100. Change the value of -First if you need more.
Get-MailboxDatabase | Get-Mailbox -Archive -ResultSize Unlimited | Get-MailboxStatistics -Archive | Sort-Object TotalItemSize -Descending | Select-Object DisplayName,TotalItemSize -First 100