I meant what I said. You can literally just setup a PG instance and it will work perfectly fine up to a few GB. At that point, you will probably start to see certain slow queries due to bad query plans. All you need to know are the basics of EXPLAIN ANALYZE, create a few indexes. That will get you to ~100GB at which point you will start to have to deploy more serious optimizations like partitioning, denormalization, etc. Once you get to multi-TB postgres instances, you will have to look at ways to horizontally scale your DB. This can be done in the Postgres world with something like Citus, but you would probably also want to look at non-Postgres based alternatives.

Yes, if you are dealing with large databases, you need to learn about... dealing with the large databases. 5GB is something that a small laptop will do.