There seems to be a lot of confusion about the maximum size of a level in UE4.

[Commander_Shepard]

@Daniel.Wenograd;610675

2km from the origin is where you start seeing precision issues in first person, while 20km is the hard coded max size you can get to in a single map. In singleplayer, origin rebasing fixes the precision issues entirely for a map as large as you want with no consequence, while in multiplayer you still need to be mindful of server side calculations not being as precise at large distances.

That ^

Also just to be clear , origin re-basing is single player (offline) only thing right now , its not implemented for multiplayer scenarios.

So to put it in perspective:

The Map size for single player games is virtually infinite if you use origin re-basing and world composition
The Map size in multiplayer is limited to 10KM from the world origin in X,Y and Z or 20km from end to end i.e a cube of 20km x 20km x 20km (origin re-basing dose not work in multiplayer)