I (re)wrote a perlscript, based on the GWstats.pl script from Rob Anderson. For GroupWise 6.5 or earlier versions.
Put all logfiles of the different Post Offices in one folder, put the path to this folder in the script and run the script. It creates an html-file with the amount of lines you want with the biggest individual emails/calendar items, sorted on size. In the list the name of the logfile it came from, the sender and recipient of the mail, date and time and the size of the email or appointment.