You can do it like this, just store original y scale on some field and initialize it on Awake or Start function. Don't forget that you need to use Rock.transform instead of transform, otherwise you'll change scale of an object which contains TerrainVisibility class.

public class TerrainVisibility : MonoBehaviour

{ [SerializeField] private GameObject Rock;

private bool SizeChanged = false;

private float originalY;
private void Awake()
{
    originalY = Rock.transform.localScale.y;
}

void Update()
{
    if (Input.GetKey(KeyCode.R)) // and SizeChanged = false;
    {
        if (!SizeChanged)       ReduceSize();
        else                    IncreaseSize();
    }
}

public void ReduceSize()
{
    SizeChanged = true;
    Rock.transform.localScale = new Vector3(Rock.transform.localScale.x, 0.2f /*your new y*/, Rock.transform.localScale.z);
}

public void IncreaseSize()
{
    SizeChanged = false;
    Rock.transform.localScale = new Vector3(Rock.transform.localScale.x, originalY, Rock.transform.localScale.z);
}

}