If you want to search for large emails sent through Microsoft Exchange. You can use the command below via powershell to identify them.
get-messagetrackinglog -resultsize unlimited -start "03/10/2015 15:05:00" -end "03/10/2015 15:20:00" | select sender, subject, recipients,totalbytes | where {$_.totalbytes -gt "1000000"}NOTE : DATE FIELD IS IN AMERICAN FORMAT.