Explore the world, meet new people and kill them for XP
A open world game gives the player the freedom to move around in a seemingly vast world and pursue other tasks beside the primary objectives.
With a large world there is always the problem that the player miss parts of the experience. It can be as simple that he runs by a abandoned tower that he wish to explore but have to run of and do a quest and then forget about the tower. For that one can place markers on the map for important places as the player detect them. As the player runs by the tower place a marker of a tower on the map. To improve the chance that the player finds them one can hint about them on the compass when the players starts to get close enough.
Even with markers there is a risk that the player never pass by the given area at all and fail to detect the marker. To avoid that one can add Viewpoints that can be used to reveal the markers around it. In Assassin's Creed there are towers that you use to see all the markers around the tower. These viewpoint often have a large detection radius or are visible from the start. As a final step in revealing the markers one can add a late game powerup that reveal all the viewpoints on the map.
With a large open world it start to take time for the player to move from one place to another. Fast travel solves this by making it possible for to player to select markers on the map and move directly to that place. The game world can still simulate that time pass by but the player will skip the trip. Not all map markers have to allow fast travel and it's often not possible if the player are in combat.
Open World Games
Here is the world size listed for some open world games.