That's a good question, thanks! I created the game itself with an extremely small fixed width and height as a way to cut down on the tight development time. (Resizeable layouts being a huge time sink to get right.)
For kind of a "retro feel" (meaning the images will get bigger, but pixelated):
You can right-click in the game area (it's an <iframe>) and "Open frame in new tab" or "Open frame in new window" - in Firefox's wording and other browsers should have similar options.
That gives you a nice clean window, but doesn't make the game any larger. *Then* you can use the browser to "zoom in" to fill the screen with the game.
It's a pretty lame hack, but might give you a bit of that immersive feeling?