Get-MailboxStatistics

Mailbox Statistics for:
MB server
Storage Group
Mailbox database

get-mailbox -Database "sch-mb02\Level 1-MB02\Level 1-MB02" |
Get-MailboxStatistics |
Where {$_.TotalItemSize -gt 400MB} |
Sort-Object -Property TotalItemSize -Descending |
Format-Table DisplayName,TotalItemSize,ItemCount,StorageLimitStatus -auto