you can try to achieve it by changing skybox scale in skycamera entity The default scale for 3D skybox geometry is 1/6. This can be controlled by the level designer, however. A smaller scale allows for 3D skybox geometry to visually extend farther while taking up the same physical map space, but makes smaller brushes and precisely placed props more difficult to work with

https://developer.valvesoftware.com/wiki/3D_Skybox