Changing start scene gui size main menu

I did some searching but couldn't find answer to this basic question.

I have done this code in a script:

 using UnityEngine;
 using System.Collections;
 
 public class Menu : MonoBehaviour {
     
     void OnGUI () {
         if (GUI.Button (new Rect (25, 25, 100, 30), "Button")) {
             // This code is executed when the Button is clicked
         }
     }
 }

Then I created an empty game object and put the script in it. When I press play then I can see the button in the corner.

But How can I change the size of the window? This is supposed to be the window that comes when they start the game... With buttons such as "play" "options" "load" etc.

It would be much easier if I could see the actual window that I have created with unity. With the border and x button in corner etc.

Hope you understand my basic question!