@simplenomad i would recommend setting up object storage with an s3 compatible provider, as that's where the 98% of the space is used. psql db is minuscule, as is the redis instance.
if you go traditional, you're probably safe with about 30GB of storage, but you'll need to ensure the cleanup scripts work, for pruning old files
@simplenomad completely configurable with tootctl on the command line and now with settings in the Admin panel, you can now set purge times for different media.
However, to your actual question, initial storage will be larger then once things get moving it tends to even out. I'm running an instance with 2 whole users, but I also relay, so my actual media storage is a bit more.
My instance has been up since June and I'm sitting with 15Gb of stored media right now.