Enter full screen on Computer without a new scene

This will do the trick

 using UnityEngine;
 using System.Collections;
 
 public class ToggleObjectScale : MonoBehaviour
 {
     private bool objectMaximised = false;
     private Vector3 norlmalScale = Vector3.zero;
     private Vector3 maxScale = Vector3.zero;
 
     void Start()
     {
         norlmalScale = gameObject.transform.localScale;
         maxScale = new Vector3(Screen.width / norlmalScale.x, Screen.height / norlmalScale.y, norlmalScale.z);
     }
 
     void OnMouseDown()
     {
         if (objectMaximised)
         {
             gameObject.transform.localScale = norlmalScale;
         }
         else
         {
             gameObject.transform.localScale = maxScale;
         }
         objectMaximised = !objectMaximised;
     }
 }

You need to make sure that there is a collider on the GameObject.

The thing is it will max the complete object. You may want to create an object that just represents the screen and attach this script to it. That way it will only enlarge the screen itself.

I did a quick test using a Quad and a sample material. Worked fine.

This is another version which is a little better

 using UnityEngine;
 using System.Collections;
 
 public class ToggleObjectScale : MonoBehaviour
 {
     private bool objectMaximised = false;
     private Vector3 norlmalScale = Vector3.zero;
     public Vector3 maxScale = Vector3.zero;
 
     public float distance = -10;
     public float goDepth = 0.3f;
     private Vector3 v3ViewPort = Vector3.zero;
     private Vector3 v3BottomLeft = Vector3.zero;
     private Vector3 v3TopRight = Vector3.zero;
 
     void Start ()
     {
         norlmalScale = gameObject.transform.localScale;
 
         goDepth = Camera.main.nearClipPlane;
         distance = Camera.main.transform.position.z;
         v3ViewPort.Set(0, 0, distance);
         v3BottomLeft = Camera.main.ViewportToWorldPoint(v3ViewPort);
         v3ViewPort.Set(1, 1, distance);
         v3TopRight = Camera.main.ViewportToWorldPoint(v3ViewPort);
         maxScale = new Vector3(v3BottomLeft.x - v3TopRight.x, v3BottomLeft.y - v3TopRight.y, goDepth);
     }
 
     void OnMouseDown()
     {
         if (objectMaximised)
         {
             gameObject.transform.localScale = norlmalScale;
         }
         else
         {
             gameObject.transform.localScale = maxScale;
         }
         objectMaximised = !objectMaximised;
     }
 }