mozilla-b2g/marionette-b2gdesktop-host — pull request #11 by cctuan (2013-11-11 09:27:15+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
}});