I have a similar problem with my triple monitor setup (Full HD – 4K – Full HD monitors) as shown below:
As you can see, when I toggle full screen preview, the video is only about 2/3 of the monitor size. I’m pretty sure that Windows item scaling may have to do something with it, as I have it set to 150%. Premiere also has a huge problem with timeline zooming using scroll wheel on 4K monitors, but thats a different problem.
Anyway, since I couldn’t find any solution, maybe this workaround will help you:
I noticed when I alt+tab switch to any other application during my fullscreen preview, and then go back to premiere, the image magically becomes actually full screen size. The faster method to achieve this is just pressing the start button (if you’re on windows 10 like me), which “deactivates” premiere and sets the start menu as active window, and then pressing it again to make the menu disappear, which sets premiere as an active application again. It’s a pretty quick way to get what we want, as it’s just two taps more – the fullscreen toggle key, and then windows key twice.
Let me know if the trick works for you! 🙂