I’ve been testing a custom untiled 256km2 world in multiplayer (without objects, just vegetation) without any problems. It’s about the same size as the map for NSF:Rivals.
That’s a lot bigger than DayZ, and almost overkill in my opinion. Unless your game has either driving or flying vehicles I would stick something between 8x8 and 16x16km2.
I still need to work out support for tiled maps though. Has anyone tried disabling world origin shifting alltogether, like described in the docs?