This Exchange management shell command will reach out to all your hub transport servers and then search through the tracking logs for messages greater than a given size. Handy if you want to track down larger messages moving through your Exchange environment. This particular example searches for stuff greater than 50MB.
Get-ExchangeServer | where {$_.isHubTransportServer -eq $true} | Get-MessageTrackingLog -resultsize unlimited -start "8/1/2010 12:00AM" -end "8/4/2010 11:59pm" | where {$_.TotalBytes -gt 50000000}
