Microsoft Exchange 2007 will not allow you to check Mailbox sizes unless you access each mailbox one at a time. To list mailbox sizes, do the following:

1) Access your Exchange Server
2) Open Exchange Management Shell
3) Type or copy the below command and press enter:

Get-MailboxStatistics | Sort-Object TotalItemSize -Descending | ft DisplayName,@{label=”TotalItemSize(MB)”;expression={$_.TotalItemSize.Value.ToMB()}},ItemCount

You can now select all and copy to a notepad to analyse the data