In Spaceclaim, you must change the Units before you start to create any geometry to shift the geometry engine to the smaller domain. This dialog box is found under the File menu.
After you have created some geometry, the smaller domains will not appear in the drop down list.
Consider using a Surface Coating to represent the 10 nm thick layer on the surface of the 400-micrometer box.
In the image below, I choose Micrometers from the list above and have defined a 1 nm coating.
By using a surface coating, there is no need to create a thin layer of uniform thickness geometry to mesh with solid elements. Surface coating will apply the material at the correct thickness to one ore more faces of the solid body.