How could watching playback in full screen be so hard in Premiere??? It's admittedly stupid that Premiere have not figured this out yet. In Media Composer you just press command + shift + F and it never fails.
In Premiere is supposed to be the grave accent key, but what it does is turn whatever window I have activated into full screen. I've spent 20 minutes googling this and my conclusion is that going back to Media Composer is the best I could do.