Using Exchange Powershell to report on Mailbox Sizes 
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Database Size
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Get-MailboxDatabase -Status | select name,databasesize,availablenewmailboxspace


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Mailbox Size - Drops to the csv on the C drive
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Get-MailboxStatistics -Server 'MBX-SERVER' | Select DisplayName, ItemCount, TotalItemSize | Sort-Object TotalItemSize -Descending | Export-CSV C:\MailSize.csv



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Show top ten Mailbox folders with most Items on the Mailbox Server
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Get-Mailbox Server 'MBX-SERVER' -ResultSize Unlimited | Get-MailboxFolderStatistics | Sort-Object ItemsInFolder Descending | Select-Object -First 10 | FT Identity,ItemsInFolder,FolderSize Wrap


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Top Ten folders in one users Mailbox by size
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Get-Mailbox -Identity "username" -ResultSize Unlimited | Get-MailboxFolderStatistics -FolderScope All | Sort-Object FolderSize -Descending | Select-Object -First 10 | FT Identity,FolderSize,ItemsInFolder wrap