How Do I Change the Width of the Template?

Have you edited /custom/template_info.php to reflect your template's name etc.? Until you do that, Zen Cart won't recognize your template.
Are there two "Classic" template selections? If so, one of them is yours, without the right name.

Thanks Glenn. I just named the custom template CUSTOM like in the instructions.

The idea of that part of the instructions is that you put your own preferred template name wherever you see CUSTOM. But if you've used CUSTOM that's not a big deal.

More important is that you actually select your new template in Admin > Tools > Template Select. Otherwise any changes you make to it will no effect whatsoever on your site.

And, as Glenn has said, you should edit your template's template_info.php file to reflect your template's new name so that you can tell it apart from Classic when you try to select it.