Hi Michael,
While exporting the message tracking results, it limits the data to only provide the message information on date, MID, Host, sender, recipient, subject and the last state.
There is another way of retrieving the message size in reports is via GUI -->Email --> Reporting--> System capacity --> Incoming/Outgoing Mail.
Choose the time range accordingly. At the bottom of the statistics you will find the Total Incoming/Outgoing message size in bytes.
Note: This report will contain only the time stamp and the message size in bytes for that time period. This may not contain the exact information on each of the emails associated with its MIDs.
I hope this helps.
Regards,
Anjali