I upgraded to Firefox 31 today and I noticed a new feature (it may have been there a while? but it's the first I noticed it).
So, enjoying making my latest example of AngularJS and Worklight I was doing a Preview when I forgot that I'd switched my default browser to Firefox.
and if you want to cater for a CUSTOM size then you can move these about and make it fit.
Obviously, if using Worklight Studio...I could just do a Preview of the app using the Browser Simulator...
...and you can have multiple on the screen at once.