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.