Another great tip from Lifehacker.com:

You can quickly change the size of the Firefox window by clicking a bookmarklet. Reader Domen writes in and explains how to set up the bookmarklet, which is perfect for users with extra screen estate. Create an empty bookmark in your Bookmarks Toolbar and enter the following as the location:

javascript:window.resizeTo(1280,1050);

Edit: Well, I was going to be nice and post pre-made bookmarklets for you, but WordPress strips any JavaScript links of their magic. :-P