Screen resolution of Googlebot mobile?

The current GoogleBot Smartphone agent, as tested with the 'Fetch as Google' Tool is essentially a fake iPhone using a headless Webkit Engine, running on a Linux x86_64 desktop machine.

The default non-responsive viewport width is that of an iPhone at 980px.

With a viewport <meta name="viewport" content="width=device-width, initial-scale=1"> applied, it has a Screen Size and CSS Pixel Display Resolution of an iPhone 5s at 320x568** pixels with a 2x Device Pixel Resolution emulation.

NB: The Engine does not support Touch nor window.orientation like an emulator would.

UPDATE: *As of Dec 2015, the bot has now been updated to a viewport size of 375x667 pixels (size of an iPhone 6) and the engine is now using a Pre-Blink QtWebKit engine with a v8 JavaScript engine.

UPDATE 2: **As of April 18 2016, The size has changed again for a Nexus 5X screen size of 411x731 pixels and a new user agent. Noting that 411x731 is the screen size info. But the actual viewport size (clientWidth and clientHeight) are reported as 410x730px, with an emulated DPR (devicePixelResolution) of 2.625. The platform's given architecture is armv8l.

UPDATE 3: As of April 18 2017, it has switched to the blink engine. Same dimensions, resolution and platform. UA unchanged.

Google's bots do not view a page in any resolution due to the fact they do not render the page as you would expect from a browser, therefor resolution is irevelant but you should aim for standard media queries that support a range of devices. Google's mobile bot will view the media queries you have in the CSS file and estiblish from the Max-width resolution what devices your site works.

So the only thing you need to know is how the media queries work, if your have a min-width:300px and a max-width:600px as your media query Google will be able to tell that you support iphones, blackberry and smartphones.

SOURCE

The max-width value of 640px shown above is an example, not a requirement. Our algorithms look for max-width values that can be reasonably expected to refer to smartphone screen resolutions, and we will try to monitor what typical mobile websites use and may update our algorithms accordingly in the future.

JavaScript

Ideally your pages should render completely without the use of JavaScript using no-js, Google is pretty limited in what it understands, Google can use CSS to understand media queries display:none on desktop and mobile but as far as detecting your JavaScript its impossible to say. Personally as I said I'd ensure that the page renders without JS and then just serve the same version for both mobile and desktop when JS is disabled.