I launched Blender, in the default "Startup file". I changed the Camera Location X, Y and Z, halving them.
Then I changed the "Units" in the "Scene" to Millimeters as you can see in the following screenshot; in the same screenshot you can see the cube "Dimensions" is 2mm.
The rendered image is fine:
But I do not understand the mutual size of the cube and the camera Focal Length: the cube side length is 2mm and the focal length is 35mm, so the focal length should be ~17x longer than the cube side but in the following screenshot the focal length seems shorter than the cube side.
What am I missing?