How do I create a test window with set dimensions?

I am building an HTML5 app and I have been using FuncUnit for my unit tests. Now I would like to create tests specific to a smartphone layout - about 480px width. How can I create a new window and run FuncUnit tests in it? I tried S.extend( FuncUnit ) for FuncUnit.open and FuncUnit._open so I could modify their internals in steal.then( function() { }) but that hasn't worked.