Generate Mailbox Size and Usage Report using PowerShell for Exchange 2010 / 2013
An easy way to get a mailbox usage report is via the Get-Mailboxstatistics commandlet in powershell and a combination of other commands. The below script will output a mailbox usage report in HTML format:
Get-Mailbox -ResultSize Unlimited | Get-MailboxStatistics | Select DisplayName,TotalItemSize,ItemCount,Database,LastLogonTime,LastLoggedOnUserAccount | SortTotalItemSize -Descending | ConvertTo-Html -Title "Mailbox Stats"| Out-file "C:\MailboxStats.html"