+1
for anyone running into this in the mean time, this method works:
const chromeless = new Chromeless({ remote: true, viewport: {width: 500, height: 500} })
It's due to the protocol not exposing a great way to change size. Responsive sites seems to get updates when you forceViewport, but other sites don't. I've tried a few different strategies, but it's something we'll likely need at a lower level in Chrome, or find some JavaScript "hack" to trigger it...
Most helpful comment
for anyone running into this in the mean time, this method works: