This pack includes all models in the database.
This pack excludes models with a polygon count above 1500. Old versions of models are also excluded.
1500 was chosen to limit the total player polys in a server to about 100k (32 players + 32 corpses). Performance issues begin at around 50k on modern hardware but it should be uncommon to see 32 players/corpses in one area, as well as for all players to choose a 1500 poly model.
Extract either pack to valve_downloads, found here:
Steam/steamapps/common/Half-Life/valve_downloads/
You may not want to extract the sound folder. Model sounds sometimes trigger rapidly and drown out every other sound in the game.