There are a few things to consider if you want to view your visual in Presentation Mode in fullscreen.
If you'd like your visual to automatically be visible in fullscreen when in Presentation mode, you'll first need to Publish it.
Next, using the output URL provided, you can access Presentation Mode from the output page.
Then click the 'present' button for your browser to turn fullscreen (may need to wait 2-4 seconds) and ensure your browser is zoomed to 100%.
If you try accessing full screen Presentation Mode from the dashboard by clicking "Present" there, the output page will open Presentation Mode right away but not full screen. When this happens, the presentation will show as usual but the browser won't go into fullscreen by itself. To clarify, this is a security feature offered by most modern browsers- they cannot go into Fullscreen mode unless it's initiated by the user.