Unfortunately:
ESA cannot report the final message size in tracking if the message was rejected during SMTP DATA phase due to size.
At that point, the appliance has only seen:
Envelope data (MAIL FROM, RCPT TO)
Partial or no message body
It does not know the final size because it intentionally stopped accepting data.
Enable more verbose SMTP logs
Go to:
Monitor > Logs > Log Subscription > mail_logs
or configure logconfig in CLI.
This logged size is approximate, based on what has been received so far before aborting.