If you're using websitebuilder keep ALL content between the vertical lines on the sides of the page your editing.
If you're using sitebuilder plus or desktop you'll need to adjust the page width. I suggest you go to your visitor stats/technology/screen resolution to see what screen size is most common among your visitors and adjust the page widths accordingly.
When building your website in WebsiteBuilder you want to keep the content in between the vertical dotted lines on the pages. This section of content is responsive and will adjust to fit the size of the monitor viewing the site.