Mana Bar Help: Decreasing the size of a GUI.Box on click

Hi there, I have a question. I am trying to decrease the size of a GUI.Box on click, and maybe it refills after a couple of seconds. Here is what I have now. I have no idea where to begin:

#pragma strict

var ammo : float;
var fireRate : float = 0.5;

private var nextFire : float = 0.0;
private var shooting : boolean = true;
private var flames : boolean = true;

function OnGUI() {
	
	 GUI.color = Color.red;
	 GUI.Box(Rect(20,200,Screen.width / 5,Screen.height / 20), "" + ammo);
		if(Input.GetMouseButtonDown(0) && Time.time > nextFire){
			nextFire = Time.time + fireRate;
				ammo -= 1;
					GUI.Box(new Rect(20,200,Screen.width / 7, Screen.height/20), "");
			{
				if(ammo < 0)
					shooting = false;
					flames = false;
					yield WaitForSeconds(5);
					ammo += 1;
				
				if(ammo > 0)
					shooting = true;
					flames = true;
			}
		}
	
	if(shooting == false){
		
		GameObject.Find("Actual").GetComponent(MagicShoot).active = false;
		GameObject.Find("small flames").active = false;
	}
	
	else
	{
		GameObject.Find("Actual").GetComponent(MagicShoot).active = true;
		GameObject.Find("small flames").active = true;
	}
	
}

use a set variable instead of Screen.width / 7