From LedHed's Wiki
Get-MailboxDatabase -Status | Select-Object Name,@{N="DatabaseSize GB";E={"$([math]::round($_.DatabaseSize.Tobytes() /1Gb, 2)) GB"}},AvailableNewMailboxSpace | Sort-Object Name | fl
Reference
http://www.experts-exchange.com/Software/Server_Software/Email_Servers/Exchange/Q_28282182.html