var controller : CharacterController;
   var oldHeight;
   var flagStand = false;
   var dist;

   function Start ()
   {
        controller = GetComponent(CharacterController);
        oldHeight = controller.height;
        dist = oldheight / 4 ; //вообще - (oldheight - controller.height) / 2, но т.к.  controller.height == oldheight / 2, то получаем это
   }

   function Update()
   {
        if(flagStand && !Physics.Raycast (transform.position, Vector2.up, oldheight / 2))
        {
           StandUp();
           flagStand = false;
        }
        if (Input.GetKey(KeyCode.LeftControl)) Squatting();
        if (Input.GetKeyUp (KeyCode.LeftControl) )
        {
           if(!Physics.Raycast (transform.position, Vector2.up, oldheight / 2)) StandUp();
           else flagStand = true;
        }
   }
   function StandUp()
   {
        controller.height  = oldHeight;
        transform.position.y = transform.position.y + dist + 0.01f
   }
   functoin Squatting()
   {
        controller.height = controller.height / 2;
        transform.position.y = transform.position.y - dist + 0.01f;
   }

RaycastHit toGround;   
isGround = Physics.Raycast(transform.position,-transform.up,out toGround,totGround));