Set window size for Velocity client side integration testing

I have started using velocity and jasmine to create some client side integration tests for a small app I am working on.

What I would like to be able to do in this integration testing is to make sure the correct components inside my template have loaded given a particular screen size - to test the responsiveness of it.

For example, I have a template for a header with a logo and nav menu inside it, to be displayed on desktop devices. I also have another template for a different header and nav menu to be displayed on mobile devices.

Is there a way I can somehow resize the PhantomJS window and check to see if the correct html templates have loaded up, or an I going about this the wrong way?