almost every tutorial i see makes some mistakes in explaining it to people.
Not that their method is wrong, but it emphasises the non important while missing out the critical issues.
1. The sky camera entity always projects the skybox to the 0,0,0 coordinate, so the playable map must be built around the 0,0,0 coordinate. So if you had a river valley map then it would be an excellent idea for the river to be a coupla hundred units below the 0,0,0 point.
2. Reference geometry is used to help you make sure that the 3dsky is projected seamlessly into your level, by picking some large prominent brushes and creating a camera at the 0,0,0 point you are ensuring that the camera in the 3dskybox is accurately lined up with your level. see #1 for the reason why it is important that it is lined up.
3. your skybox scale factor defaults to 16x, contrary to what some people ahve said in this thread there is no reason that you can not change this scale factor as you see fit. A smaller scale factor would allow you to have more detail (remember the grid minimum is 1 unit, which in a 16x skybox would translate to 16units!) As for the skybox models, thats entirely up to you when you make models, or if you are using someone elses models you can use the tutorial in the modelling section to rescale them.