Hello, how to change the Width and Height in new gui:

I tried:
using UnityEngine;
using UnityEngine.UI;
using System.Collections;
public class RectTest : MonoBehaviour {
public Button MyButton;
void Start () {
MyButton.height = 700; //not working
MyButton.transform.height = 700; //not working
MyButton.transform.position.height = 700; //not working
MyButton.gameObject.transform.position.height = 700; //not working
}
}
Thanks!