It doesn't actually include all the media, and -- I think -- edit history. It does give you a decent offline copy of the articles with at least the thumbnails of images though.
Edit: If you want all the media from Wikimedia Commons (which may also include files that are not in Wikipedia articles directly) the stats for that are:
Total file size for all 126,598,734 files: 745,450,666,761,889 bytes (677.98 TB).
according to their media statistics page.