Screen height and width confusion

You need to flip height and width like this and also offset the position by half the width (because the box pivot is at a corner, not the center):

 using UnityEngine;
 
 public class Test : MonoBehaviour
 {
     void OnGUI()
     {
         Rect rect = new Rect(Screen.width / 2f, Screen.height / 2f, 120f, 60f);
         rect.x -= rect.width / 2f;
         rect.y -= rect.height / 2f;
         GUI.Box(rect, "Box in the middle");
     }
 }
 

Maybe you can think of it like this as well:

 Vector2 size = new Vector2(120f, 60f);
 
 Vector2 position = new Vector2(
     x: Screen.width / 2f - size.x / 2f,
     y: Screen.height / 2f - size.y / 2f);
 
 GUI.Box(new Rect(position, size), "Box in the middle");

Also be aware that the this is the legacy GUI system and is replaced by the new Unity UI system. The new system makes layouting much easier because it uses anchors and layout groups which do the work for you. It also has better performance than the legacy immediate mode GUI.

Scaling in the legacy GUI system was more of a hassle. If you want your box to stay the same size across multiple resolutions, you either have to calculate the position and size of every element manually by factoring in the screen size or edit the GUI.matrix like this:

 Vector2 referenceResolution = new Vector2(1920f, 1080f);
 Vector3 scaling = new Vector3(
     x: Screen.width / referenceResolution.x,
     y: Screen.height / referenceResolution.y,
     z: 1f);
 GUI.matrix = Matrix4x4.TRS(Vector3.zero, Quaternion.identity, scaling);
 
 Vector2 size = new Vector2(1920f - 50f, 260f);
 Vector2 position = new Vector2(Screen.width / 2f, Screen.height / 2f);
 position -= size / 2f;
 
 GUIStyle style = new GUIStyle(GUI.skin.box);
 style.fontSize = 30;
 GUI.Box(new Rect(position, size), "Box in the middle", style);

This will ensure that on 16:9 screens, the box fills the screen. But, you can also see that it's a difficult topic, because its still strongly connected to your reference resolution, so it doesn't adapt between portrait and landscape resolution, etc. All of these problems are solved by the new UI system, which was introduced with Unity 4.6, though.