I can set the viewportSize to get small mobile layout & sized screenshots:
// 1080 x 1920 is Nexus 5 resolution
casper.options.viewportSize = {width: 1080 / 3, height: 1920 / 3};
However the DPI is low, unlike when viewed on mobile.
My page uses the viewport meta tag to control layout on mobile browsers like so
<meta name=viewport content="width=device-width, initial-scale=1, maximum-scale=1.0, user-scalable=no">
What we really need is to set the Device Pixel Ratio. It appears as of July 2016, PhantomJS is lacking that ability, however people are working on it.
Keep full resolution, and use Casper's zoom
// 1080 x 1920 is Nexus 5 resolution
casper.options.viewportSize = {width: 1080, height: 1920};
casper.start().zoom(3)
This will probably fail under responsive designs with @media queries relative to screen size. I'm still hoping for a less hackish answer, but this at least gets us a little closer.