Help - Making Captcha txt bigger

<?php

require_once( '../inc/header.inc.php' );
require_once( BX_DIRECTORY_PATH_INC . 'images.inc.php' );

ob_start();
$chars = array("a","b","c","d","e","f","h","i","k","m","n","o","r","s","t","u","v","w","x","z","2","3","4","5","6","7","8","9");
/*
$chars = array("a","A","b","B","c","C","d","D","e","E","f","F","g","G","h","H","i","I","j","J",
			   "k","K","L","m","M","n","N","o","p","P","q","Q","r","R","s","S","t","T",
			   "u","U","v","V","w","W","x","X","y","Y","z","Z","2","3","4","5","6","7","8","9");
*/

$textstr = '';
for ($i = 0, $length = 6; $i < $length; $i++)
   $textstr .= $chars[rand(0, count($chars) - 1)];

$hashtext = md5($textstr);
setcookie('strSec', $hashtext, 0, '/');

if ( produceSecurityImage( $textstr, $hashtext ) != IMAGE_ERROR_SUCCESS )
{
	// output header
	header( "Content-Type: image/gif" );
	
    header( "Expires: Mon, 26 Jul 1997 05:00:00 GMT" );
    header( "Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT" );
    header( "Cache-Control: no-store, no-cache, must-revalidate" );
    header( "Cache-Control: post-check=0, pre-check=0", false );
    header( "Pragma: no-cache" );
	
	// output error image
	@readfile( $dir['profileImage'] . 'simg_error.gif' );
}

ob_end_flush();
?>