Hi Nikhil,
The size of messages is available in the Runtime Workbench -> Performance Monitoring.
You can get an aggregated view of the messages passed through the Integration engine for a specified period. The results for this monitoring also record the size of messages.
http://help.sap.com/saphelp_nwpi71/helpdata/en/9e/6921e784677d4591053564a8b95e7d/frameset.htm
Regards,
Alka.