I'm running it in the smallest VPS of vultr with 25GB of disk.
This instance only has 3 users, with me being the only active. It says it's been up for almost a month and I've only used 3GB.

Here are the docker volumes which have the actual data of your instance, and from inside the DB the biggest table is the one called activity which the devs said it's only sometimes used to validate the data, but could be truncated if needed (there's a schedule task which only keeps up to 6 months).
Also the thing to have in mind is to properly configure the logs of whichever installation guide you follow.
After that I've seen other admins say the next biggest is the media uploaded (from bigger instances).

$ du -h --max-depth=1
640K    ./pictrs
3.2G    ./postgres
3.2G    .

lemmy=# select
  table_name,
  pg_size_pretty(pg_relation_size(quote_ident(table_name))),
  pg_relation_size(quote_ident(table_name))
from information_schema.tables
where table_schema = 'public'
order by 3 desc;
         table_name         | pg_size_pretty | pg_relation_size
----------------------------+----------------+------------------
 activity                   | 2187 MB        |       2292867072
 comment                    | 56 MB          |         58212352
 person                     | 48 MB          |         50307072
 comment_like               | 45 MB          |         47161344
 post_like                  | 22 MB          |         22781952
 comment_aggregates         | 14 MB          |         14811136
 post                       | 13 MB          |         13623296

Well, here's my first post on the fediverse!

Background in IT and server administration here. I however do not know much about the intricacies of the fediverse, but am interested in learning. Here's my two cents based on a background of LAMP stacks for web hosting.

The required space would likely scale and vary greatly depending on how much content is hosted locally. Assuming minimum space similar to a basic LAMP server it'd likely have starting space requirements of less than 1GB. If local content is primarily text/links to content hosted elsewhere it would take a lot to drastically change that space requirement. Image hosting can vary greatly depending on size, quality, and number of images. Video hosting is an absolute space hog even at fairly low resolutions by today's standards.

Bandwidth requirements would scale similar to storage requirements.

Other specs would also start very low if fediverse requirements are similar to a LAMP stack. Cores are typically more important than core speed in web server hosting as each request will try to use a separate core, but doesn't need much processing power to provide that request since the server isn't actually rendering anything.

Likewise, you shouldn't need much memory on a web host. Will scale with the number of scripts running on the host but I suspect that shouldn't be many unless you're also running moderation bots, but those should ideally be run on a different server instance.

That said, I'd also be curious to hear from other people that have experience with the fediverse though and other recommended specs to potentially host an instance.

If anyone has other questions I'm happy to try to help :)