I tried playing in the browser first, and while I could change the screen size, it just always said it was wrong no matter what I did. Thought it was a browser issue, so I tried the downloaded version. In my experience, downloaded versions work better if the browser version has issues... but in this case, you can't change the screen size at all, so it's even worse than the browser version.

EDIT: Ok, I figured it out. It appears the "wrong screen size" popup just doesn't calculate screen size in real time, so you have to close it and retry after changing your screen size.