Custom Width Blank Page

I’m looking to move my website over to WordPress and have selected the Responsive theme. I would like to create a custom page size that does not have margins.
Is the best approach to create a custom page template, ‘Custom_Page.php’, or should this be done somehow using a Child Theme?
If you have any examples that show how others have done this, that would be a huge help for me. In advance, thanks for your suggestions and advice.