game screen dimensions

Everything about Screen: http://docs.unity3d.com/ScriptReference/Screen.html

Current Screen Resolution:

  void Example() {
         print(Screen.currentResolution);
     }

Getting and setting screen resolution:

 void Start() {
         Resolution[] resolutions = Screen.resolutions;
         foreach (Resolution res in resolutions) {
             print(res.width + "x" + res.height);
         }
         Screen.SetResolution(resolutions[0].width, resolutions[0].height, true);
     }

So you can get width and height values, set new ones on specific dimensions like 4:3, 16:10 etc.