Hi moodle question, the first thing you need to do is find out how big the files are in the courses you are going to upload. This data is the biggest factor.
Then add on about 200MEG for Moodle core files.
If you are allowing private files you will need to allow for this as well.
Maybe a bit more if you intend having lots of users.
Then there is the problem of space for backups. If one file is linked in several courses, it is backed up in each course.