If you are creating a map from scratch, it's best to make the 3D skybox details in the original size first, place them in the places where they should be visible, put the sky_camera entity at the center of the world. Then, just select all the 3Dsky geometry along with the sky_camera, reduce their scale by 16 times, and move it away from main area, and block it by making a new skybox brushes.
Here is a tutorial: