We'll add this value to the next release, thanks for your suggestion!
In the meantime, to get a reasonable estimate of "total bytes" value, save the message to an instance of System.IO.MemoryStream and use Length for this purpose. This should be very close to the desired value in most cases (but make sure to be able to handle values of BytesTransferred grater than the Length).
Update: This has been added in Rebex FTP/SSL 2012 R2.