You're basically asking for a way to resize the iframe itch.io puts on your game page. That's quite different from resizing your canvas to fit the iframe.
Not really, I'm asking to get the windows real width not the iframes width so I can resize the canvas my self. That would work though if I made my canvas the size of the iframe. That might actually be a better solution than get access to the real window width from your game.
But for now, your code is actually working just fine, I suspect. It just can't do what you expect of it.
yeah running at max res is alright. It would be working fine if worked like it did locally. Hoping this is addressed by the dev team in the future.
It looks like (by the lack of replies) that not many people know about this or are unaffected by it. So thanks for your suggestions!