I can see that the width of rdsenergyuk.wordpress.com has already been set to “Wide” via Theme Options > Website Layout in the WordPress.com Customizer.
You can further increase the amount of space that page content takes up by assigning the “Full-width, no sidebar” page template to individual pages.
To assign a page template: Navigate to the editor for an individual page and then choose from the Template dropdown menu under the Page Options module.
If the above is not enough for you, then you could further increase the width that your site’s content takes up using some custom CSS.
The following snippet will override the theme’s design and increase the maximum width that your content can take up:
Before going ahead with the above custom CSS, I recommend thinking about how the width that your content takes up will impact its readability. These are some good articles related to that:
The theme’s author will have considered the above when including the layout options.
Let me know if that information helps out or if you have any further questions.