I just tried out uploading some of my projects, and I noticed there's no way to make the window scale automatically to fit the browser window. The best option is the full-screen button, but that's not very ideal IMO.
It would be cool if it was possible to either select an option where the game iframe scales with the browser window size, or add some kind of an "enlarge" button that does it instead of the "full-screen" button.
Add this style in your browser dev tools to see what I mean:
height: calc(100vh - 150px)!important;
width: calc(100vw - 100px)!important;