Twenty Thirteen: Full width page

child theme style.css should read;

/*
Theme Name:     Twenty Thirteen Child
Theme URI:      http://sousvide.reynoldstocks.com/
Description:    Child theme for the Twenty Thirteen theme
Template:       twentythirteen
Version:        0.1.0
*/
@import url("../twentythirteen/style.css");
body .site-header {
background-size: 100%;
}
.entry-header,
.entry-content,
.entry-summary,
.entry-meta {
 margin: 0 auto;
 max-width: 1440px;
 width: 95%;
}

Copy page.php in to your child theme and rename it to full-width.php.

At the top of the new full-width.php where it reads: * The template for displaying all pages - change that to * Template Name: Full Width.

In style.css locate the elements

.entry-header,
.entry-content,
.entry-summary,
.entry-meta {
 margin: 0 auto;
 max-width: 604px;
 width: 100%;
}

copy them to your child's style.css and rename thus:

.entry-header-full,
.entry-content-full,
.entry-summary-full,
.entry-meta-full {
 margin: 0 auto;
 max-width: 1110px;
 width: 95%;
}

Back to your new full width template change those elements to match and you should have yourself a full width template which you can tweak to your needs.

Also, you may want to delete "<?php get_sidebar(); ?>" from your fullwidth.php if you want to not worry about overlapping sidebars over your full width text, it may or may not be a problem for you.