According to this thread, as of version 4.6, the maximum supported multiplayer map size out of the box is 20km x 20km. Is this the limitation you were referring to? That wouldn't accommodate an MMORPG or Minecraft, but otherwise seems quite big.
Never understood the concept of measuring map size in km. It's like measuring distance on a map while ignoring scaling. Something like source's hammer units make a lot more sense to me.
1 unit = 1cm in UE, so 20km has some value as a measurement in game.
Of course, what matters more is what you do in that 20km, but it's also a limit coupled tightly with distant mesh performance and float accuracy issues.