AutoScale Orthographic Camera

I need help to setup auto scale orthographic camera. I am new for 2D game development. I have created simple plane and applied texture on it using transparent cutout shader and made it prefab. then i instantiated plane prefab in grid pattern 3x3. I want to change this grid pattern randomly, whenever start my game, grid pattern should be random like (3x4) (4x6)(4x8).so I used random.range but the problem is, I am not able to make that grid on the center of the orthographic camera, whenever my game start, grid doesn’t come on the center. I also want it to be auto scale because it doesn’t fit on screen resolution because grid is randomly changing (3x3), (3x4), (4x5)
I really need to sort out this issue because i have no idea how orthographic camera works.

public int gridWidth =0;
public int gridHeight = 0;
public GameObject facePrefab = null;

void Awake()
	gridHeigth = Random.range(3,5);
    gridWidth =  Random.range(3,5);
for (int x = 0; x < gridWidth; x++) {
for (int y = 0; y < gridHeight; y++) {
GameObject g = Instantiate (facePrefab, new Vector3 (x, y, 0), Quaternion.identity)as GameObject;

any help would be appreciated
Thanks in advance

public float randomOne;
public float randomTwo;

Void Start()
randomOne = Random.Range(3,6);
Camera.main.orthographicSize = (randomOne / Screen.width * Screen.height / 2.0f);

not sure. but try this

if there is some type error the just (float) before Random.Range like this (float)Random.Range cuz its int right now.