I always struggle to find a straight forward command for getting Mailbox sizes in Exchange 2010. Although there is a lot of information out there, they all seem to over complicate things and don’t just give you what you require.

This is all you need:

Get-MailboxStatistics -Database “Mailbox Database*” | Select DisplayName, ItemCount, TotalItemSize | Sort-Object TotalItemSize -Descending | Export-CSV C:\MailBoxSizes.csv

This command will get the mailboxes sizes for all Databases beginning with Mailbox Database, Sort them according to size and export to a csv file.