Get Mailbox Database Size

Database Size

Get-MailboxDatabase -Status | Select-Object Name,@{N="DatabaseSize GB";E={"$([math]::round($_.DatabaseSize.Tobytes() /1Gb, 2)) GB"}},AvailableNewMailboxSpace | Sort-Object Name | fl

Get-MailboxDatabase -Status | Select ServerName,Name,DatabaseSize

Mailbox Count

Get-Mailbox -ResultSize:Unlimited | Group-Object -Property:Database | Select-Object Name,Count | Sort-Object -Property:Count –Descending

Reference

http://www.experts-exchange.com/Software/Server_Software/Email_Servers/Exchange/Q_28282182.html

http://msundis.wordpress.com/2011/10/28/quick-tip-view-the-database-size-and-mailbox-count-for-mailbox-databases/