If you are administrator (admin) or equivalent or GroupWise Administrator try this:
1.- Set your account as Manage (Librarian) of all the libraries.
Use ConsoleOne for do this.
2.- Access your mailbox through the Client GroupWise.
3.- Perform a "Quick Search".
4.- Select "All libraries".
5.- Leave the field "Search" in blank.
6.- Wait for the result of the search.
7.- When the search is complete, you will see all the libraries and all the documents stored in those libraries.
8.- If you want to know the sizes of each document, add the field "Size" to the search result.
9.- Select the entire result, make a copy and export it to a spreadsheet, now you can add the field "Size".
I hope it will be helpful.