They have no actual scale or size. Units in 3D are just like units in 2D. You can create assets, where 10 pixels on the screen cover one cm in reality and you can do the same so that it covers 1 or 100m. In 3D, it additionally depends on the camera settings. Personally, i prefer to think in steps of 10 in 3D, which works fine for most indoor scenes with the default camera settings.
Just play around with these things to get a grasp of it.