mozilla-b2g/mozilla-runner — pull request #3 by cctuan (2013-11-11 10:21:07+00:00)
Title: Bug 930948 - [Flatfish] marionette test on tablet

https://bugzilla.mozilla.org/show_bug.cgi?id=930948

we can assign the screen size as below.
var client = marionette.client({
screen: {
width: 1280,
height: 800
}});