The official manual for osm2pgsql, which is the main importer for creating PostgreSQL / PostGIS databases, is here:

I strongly recommend you to skip the section about the old “Pgsql output”, as this option will be removed in the near future from osm2pgsql, and focus on the new “Flex output” which is more powerful and flexible in database layout. And to run a recent version of osm2pgsql as there have been major developments in the past four years.

There is some guidance as to needed hardware for Planet in the manual, but if you intend to process even bigger datasets and / or be prepared for them due to future growth of OpenStreetMap (e.g. Facebook Daylight distribution of OpenStreetMap, which unfortunately is also going to disappear in the near future), then some statistics and hardware specs can be found here:

https://wiki.openstreetmap.org/wiki/Osm2pgsql/benchmarks