Execute this in EMS (Adapted from here):

$(Foreach ($mailbox in Get-Recipient -ResultSize Unlimited  -RecipientType UserMailbox){
$Stat = $mailbox | Get-MailboxStatistics | Select TotalItemSize,ItemCount
    New-Object PSObject -Property @{
    FirstName = $mailbox.FirstName
    LastName = $mailbox.LastName
    DisplayName = $mailbox.DisplayName
    TotalItemSize = $Stat.TotalItemSize.value.ToBytes()
    ItemCount = $Stat.ItemCount
    PrimarySmtpAddress = $mailbox.PrimarySmtpAddress
    Alias = $mailbox.Alias
    }
}) | Select FirstName,LastName,DisplayName,TotalItemSize,ItemCount,PrimarySmtpAddress,Alias | 
Export-CSV C:MailboxReport.csv -NTI