You can easily add a Fullscreen button anywhere in your WordPress site. Native Fullscreen is free WordPress plugin by Webmaestro.fr. The plugin allows you to show a native fullscreen button which when clicked shows true fullscreen mode of your website. Without flash. It provides you a widget and a shortcode for displaying fullscreen button. The plugin uses default CSS of your WordPress theme and hence its button matches to your website.
Installation & Usage: Install and activate Native Fullscreen plugin. After activation visit your admin area Dashboard -> Appearance -> Widgets page and drag its widget to a widget area. That’s all, go to the frontend of your website and see how it works.
Fullscreen button in Twenty Thirteen WordPress theme:
Fullscreen mode, after the button is clicked:
OR Alternatively, You may use its shorcode inside your post, page, custom post type etc.
[fullscreen target="THE-CSS-SELECTOR" text="The button's text"]