Following on from a previous article on how to find out mailbox sizes in Exchange 2007 found here, the syntax is a little different for Exchange 2010. Below are a few scripts that will help with Exchange 2010 reporting.
Get-Mailbox will show you mailbox name, alias, server and prohibit send quota. It’s a start, but no information on mailbox sizes.

Using the first script from the Exchange 2007 post you need to add the -server switch, Get-MailboxStatistics -server SERVERNAME | Sort-Object TotalItemSize -Descending | ft DisplayName,@{label=”TotalItemSize(MB)”;expression={$_.TotalItemSize.Value.ToMB()}},ItemCount to get the following report.

By adding a redirect you can output the report details to a text file.
Get-MailboxStatistics -server SERVERNAME | Sort-Object TotalItemSize -Descending | ft DisplayName,@{label=”TotalItemSize(KB)”;expression={$_.TotalItemSize.Value.ToKB()}},ItemCount > c:\reports\mailbox_sizes.txt

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