im making a open world multiplayer zombie survival game and i was wondering how many units i need to get 270 Squared Kilometers into UE, is the engine even capable to have that much land in one map?
As far as I know the maximum landscape in UE4 4.7 is 20kmx20km. 270km2 = 16.43km x 16.43km. So it should fit 
1 unit in UE = 1cm. With some simple math you can figure out how many units you need
But you are gonna want to look at world composition(World Composition User Guide | Unreal Engine Documentation) and perhaps even combined with streaming levels.
Remember you can also use level streaming / world composition as well, so in theory, the size of the world is limited to how much space you want the game to take up on someone’s machine.
Thanks for the help guys, i know this is almost the exact same size as arma 3’s altis map but its going to have cars and helos