Great! The documentation says “Scale: 1 Unreal Unit = 1 centimeter (cm)” for static meshes and skeletal meshes. No word on the terrain though. But I am assuming the same unit of measurement applies for terrain too?
Look at your level in top view, hold&drag middle mouse button and you’ll see the measuring tool. You can measure your landscape and see its dimensions. There is also a widget in the bottom left corner that gives you a distance reference as you zoom in and out.
Here is a video tutorial that I did a while ago that covers how to use the ruler. It was made using UE3 but the way that you use the ruler is still the same.