If you want to do it to specific pages, your best creating a page template and adding it to the pages you’d like to be full page.
If you’d like it for every page.
You can either;
Change your css for your site. Changing your content area to the width you’d like.
or
Edit page.php template to how you’d like it to look.
or
If its a small site and you’d like to save time.
Add a different css file to be called only on pages by putting this in your header.php
<?php if (is_page()) {?>
<link href=”stylesheet-for-pages.css” rel=”stylesheet” type=”text/css” />
<?php } else {?>
<link href=”stylesheet-for-site.css” rel=”stylesheet” type=”text/css” />
<?php } ?>
For specific pages you can call an array of pages like this:
<?php if (is_page(array(‘page-slug-here’,’page-slug-2-here’))) {?>
<link href=”stylesheet-for-pages.css” rel=”stylesheet” type=”text/css” />
<?php } else {?>
<link href=”stylesheet-for-site.css” rel=”stylesheet” type=”text/css” />
<?php } ?>
Sorry if I’ve missed the point here. Just let me know If you’d like to know something different.