Below script can be used to find the size of spam  Mails folder in the User mailbox for the given list of users. Export list of users from the server throught below command and input into the Name.txt and Execute 

Get-mailbox | Where{$_.Servername -eq “Servername”} | select-object Alias > c:\Names.txt

 
$UserList = Get-Content “C:\Names.txt”
foreach($user in $UserList)
{
     $Spamsize = get-mailboxfolderstatistics $user | Where {$_.Name -match “Spam”}  | select  FolderAndSubfolderSize
     $output = $user + ” ” + $spamsize.FolderAndSubfolderSize/1mb
     Write-output $output 
}