I personally use two different google urls with this plugin which I assign shortcuts to. I assign ctrl+I to the link that is both what dwanthny linked back to and I mentioned is included in the latest plugin version, which is google images > 400x300.
I also have another shortcut of ctrl+shift+I for a url which specifically looks for images of size 510x680 (the Overdrive size ones). I don't like the aspect ratio, but you can sometimes find a consistent set of images that are hard to pick out with the way google sorts images by default. Then I use a keyboard shortcut driven launch of Paintshop Pro via the Open With plugin to resize the image to the "correct" aspect with a couple of keystrokes.
What I *really* want but havent had the time to write yet is a "resize cover" plugin. It should let me configure multiple default sizes that I want to resize to and can assign keyboard shortcuts to. This would avoid the need to launch Paintshop Pro in the above workflow. Maybe one day...