I recall something like an 8gb download that uncompressed to a postgresql db.

The thing benchmarked in my other comment loads an OSM dump into PostGIS (in 1 particular format, translating the OSM data from an editing/versioning friendly format to something more useful for map making as it goes). The current full planet dump is 84 GB:

https://planet.openstreetmap.org/

8 GB gets you a large region still, depending on how much mapping has been done:

http://download.geofabrik.de/