By way of calibration, I have two small forums:
Smaller forum runs with 10G used of a 19G disk, backups are 200M each, 2000 posts in 200 topics, 30 users. That’s a little under 2 years of activity.
Larger forum runs with 16G used of a 25G disk, backups are 700M each, 8000 posts in 1000 topics, 300 users. That’s a little over 2 years of activity.