Bit late, but you click on the game bar so that it is targeted and then press alt+enter.
So I have the same issue except I fiddled around and somehow made it slightly bigger so I can see the game, sort of, but I can't make it bigger or maximize it properly. When I tried pressing alt+enter it just showed this:
And in reality, it's only an eighth of my screen. I don't know if anyone else could give an answer because I've been searching around and everyone else with the same issue can't seem to find an answer either.