Assigning Archive db size centrally

Hi

  I have created separate database for archive and assigned to some users. For all users default size its showing 50gb. I want to give 7Gb size for all users by centrally. I tried below powershell command, its not giving any error... but changes it not happening.

Get-Mailbox -resultsize unlimited | where {$_.archivemailbox -ne $null} | set-mailbox -ArchiveQuota 5GB -ArchiveWarningQuota 4.5GB

 I tried by adding "" in archive database, that also not coming, it still showing 50gb for archive enabled old users also new users..

  Any suggestions please...