Your slider is showing in full width while checking in our development environment. If you are not talking about the slider, can you explain in detail which content you want to make full width with a screenshot if possible?
The width of your full-width slider background image is always determined by the browser width. Based on standard screen sizes we recommend that your images are at least 1280px wide. But for larger monitors, a safer bet would be to use an image that is 1920px wide. If that doesn’t solve the issue, feel free to ask again.