How do i change the size of a Box Collider 2D in C#?

Hi Guys, I’m a beginner. I want to adjust a Box Collider 2D to the screen size, so my player object can’t get out of screen. I used the following code, but the size doesn’t change if I hit the play button, also the checkbox to enable or disable the script isn’t available. Answers to similar questions seem not work.

 using UnityEngine;
 using System.Collections;
 
     public class Frame : MonoBehaviour
     {
     
         // Use this for initialization
     
         BoxCollider2D bc;
     
         float height;
         float width;
     
         void start() {
             height = Screen.height;
             width = Screen.width;
             bc = this.GetComponent<BoxCollider2D>();
             bc.size = new Vector2 ( width, height);
             
          }
     }