The following script outputs all mailbox names and their size in MB to a flat file (c:\sizes.txt). You can copy and paste this script into exchange management shell to output all of the sizes of your mailboxes to a text file for your review. The script also sums up all of the mailbox sizes and outputs an average.  

#——–begin script——–
del C:\sizes.txt
$totalsize=0
$totalusers=0
$databases = get-mailboxdatabase
foreach ($database in $databases)
{
$mailboxes = get-mailbox -database $database
foreach ($mailbox in $mailboxes)
 {
 $user = get-mailboxstatistics $mailbox.name
 $user = [string]$user.totalitemsize
 $user = $user.Replace(”B”,”")
 $user = [int]$user
 $totalsize = $totalsize + $user
 $totalusers = $totalusers + 1
 $user = $user / 1024
 $user = $user / 1024
 $mailbox.name | out-file c:\sizes.txt -append | out-null
 $user = [math]::round($user, 2)
 $user = [string]$user
 $user = $user + ” MB”
 $user | out-file c:\sizes.txt -append | out-null
 }
}

$totalsize = $totalsize / 1024
$totalsize = $totalsize / 1024
$totalsize = [math]::round($totalsize, 2)
$newline = “`n” | out-file c:\sizes.txt -append | out-null
$totalmailboxsize = “Total size of all mailboxes: $totalsize MB”
$totalmailboxsize | out-file c:\sizes.txt -append | out-null
$totalnumbermailboxes = “Total number of mailboxes is: $totalusers”
$totalnumbermailboxes | out-file c:\sizes.txt -append | out-null
$average = $totalsize / $totalusers
$average = [math]::round($average, 2)
$averagesize = “The average is: $average MB”
$averagesize | out-file c:\sizes.txt -append | out-null

#———————End Script—————–