Link: http://social.technet.microsoft.com/Forums/exchange/en-US/0aeec9c6-1f9f-4c11-b922-d4041a11628f/how-to-get-mailbox-statistics-with-primary-smtp-address-instead-of-display-name?forum=exchangesvrgenerallegacy

Exchange 2010 SP1:
 $Report=@()
 $mailbox=Get-mailbox –resultsize unlimited
 $mailbox| foreach-object{
 $DisplayName=$_.DisplayName
 $SmtpAddress=$_.PrimarySmtpAddress
$TotalItemSize=(get-mailboxstatistics –identity $DisplayName ).TotalITemSize
$obj=new-object System.Object
$obj|add-member -membertype NoteProperty -name ”PrimarySmtpAddress” -value $SmtpAddress
$obj|add-member -membertype NoteProperty -name ”TotalItemSize” -value $TotalItemSize
$Report+=$obj
}
$Report|export-csv c:\report.csv -notype