1 Unreal Unit is 1 cm by default. You have to model your objects in Blender accordingly. For instance the starter cube you get when you create a new file is just 2x2x2 units, which you can tell by looking at the dimensions:
So this cube will just be a 2cm cube in Unreal. You can either just make your objects bigger, or you can apply a scale when you export to .fbx:
