Here is a quick tip for you. This cmdlet generates the hundred largest user mailboxes sorted by size.
$result = Get-Mailbox -RecipientTypeDetails UserMailbox -ResultSize Unlimited | Get-MailboxStatistics
$result | Select-object DisplayName,@{label=”Total Size (MB)”;expression={[math]::Round(($_.TotalItemSize.value.ToString().Split(“(“)[1].Split(” “)[0].Replace(“,”,””)/1MB),2)}} | Sort-Object "Total Size (MB)" -Descending | Select-Object -First 100