Use the community commons package, action Misc > getFileSize. Get the list of documents, loop and increment based on the retrun value from the getFileSize action.
If I am correct you can see this in your app platform monitor / trends. Here you have the Appcontainer disk usage and Database disk usage. The Appcontainer disk usage would be all the space your filedocuments take up. But maybe somebody from Mendix can tell if my assumption is correct.