I understand your need to have an actual number, but I will most definitely stress James' comments above -- these can vary widely from issue to issue, year to year, journal to journal. So, take anything here with a very large grain of salt.
That said, I can make a few conservative guesses based on my experience from a medical journal I used to work with. They published about 50 articles per year, submitted in MS-Word, with about 3 review versions, and then published in HTML and PDF. One of the things that are difficult to account for that can take up space quickly are large image files and supplementary files (eg. video, data sets), so those can confound things somewhat. Assuming Word files are around 1MB and the corresponding publication PDF is around 2MB (both on the high side, I'd reckon), you'd be looking at about 7MB per article, which for a 6 article issue would round up to 50MB. Off the top of my head this sounds entirely reasonable, ie. growing at about 200-500MB/year, but you may want to build in additional padding.
Hope this helps,