The story I saw was that Aaron Swartz had to keep returning to the MIT campus to swap out external hard drives. I don't think it fit on a single hard drive. Based on my past use of JSTOR, I would say those millions of articles would take up a few terabytes of space at least. Maybe could be cut down to under a terabyte if you re-encoded them with JBIG2 compression and packed them inside a high-compression rar or 7z file, but it would still probably be one of the largest torrents ever if the whole collection was released in one torrent.