How to view your visualization in full-screen mode

There are two ways to preview your visualization in full-screen mode.

Click the "Preview in new tab" button in the top-left corner of the editor

 Add an /embed to the end of you r published visualization URL

What you are doing here is telling your URL to act like it is being embedded somewhere. It will then fill out all the available space it has, just as if you were to embed if in your webpage. If you embed it in your webpage, it will fill out all the available space of the container it is in. You can edit this in the export settings.