Since I'll use any excuse to not do actual work, I was playing around with the banner on my personal page after reading a comment from one of the Gawker tech guys.
The top two images are ones I used before the change to Kinja 1.2. The middle screen cap is the old recommended size of 600x180 pixels. I made the bottom image to see how resizing the image would appear.
Clearly the longer, narrower image uses more of that white space between the avatar and the search box. The image I used is about 600x40 pixels (the current recommended size is 600x80) but the tech guy comment mentioned trying 600x30 pixels.
I know most people don't care about the banner on their personal page but I thought this was interesting.