Get Shared and resource mailbox size

Get-mailboxSize.ps1

$mailboxes = Get-Mailbox -ResultSize unlimited -Filter {RecipientTypeDetails -eq "SharedMailbox" -or RecipientTypeDetails -eq "RoomMailbox" -or RecipientTypeDetails -eq "EquipmentMailbox"}

foreach($mailbox in $mailboxes)
{
  $mailboxstats = Get-MailboxStatistics -Identity $mailbox.UserPrincipalName | Select-Object DisplayName, Identity, TotalItemSize
  $mailboxsizeLine = [PSCustomObject][Ordered]@{
    Identity        = $mailbox.Identity
    UPN             = $mailbox.UserPrincipalName
    DisplayName     = $mailboxstats.DisplayName
    TotalItemSize   = $mailboxstats.TotalItemSize
    MailboxType     = $mailbox.RecipientTypeDetails
  }
  $mailboxsizeLine | Export-Csv -Path mailboxsize.csv -Encoding UTF8 -NoTypeInformation -Delimiter ';' -Append
}