I've attached a diff that shows one way you might accomplish zooming the elements on the page during a feature test. I watched this test run and I'll admit that the view looks a little weird when zoomed in. It does not have exactly the same visual effect as if you changed the resolution of your monitor. I couldn't figure out how to make selenium change the zoom of the entire browser. However, all of the text elements' fonts are clearly 25% larger and the feature test fails when I include this code so I'm hoping this will be sufficient for you to write a test that covers us against larger scale resolution monitors.
Let us know if this helps or how we can further help.