Creating a full screen page

Forgive the obvious simplicity of the question but I am stuck. I am trying to update/create my very first website for our club using beaver builder and Wordpress. The original site hasn’t been touched in years. I am able to access the original page and add new items. However, the page seems to be stuck at just half of my computer screen. How do I make it contract or expand based on the computer/phone/iPad using it. Thanks for the help.