A reader wrote in to me to point out that Marc Rayman has been (frequently) quoted incorrectly as saying that Dawn had the largest solar panels of any planetary mission. Dawn's panels, at almost 20 meters tip-to-tip, are the largest that NASA has ever sent beyond Earth, but they don't beat ESA's Rosetta spacecraft, whose solar panels stretch 32 meters tip-to-tip.
That reminded me of a project I'd meant to start on some time ago, to compare the sizes of various spacecraft. Here's a few spacecraft of current interest. Compare and you can see that the real winner for solar panel size is the International Space Station; but Dawn and Rosetta are really pretty big as well. All of these images are at a scale of 20 centimeters per pixel. Poor little Spirit is barely visible at this scale.
Space station (as it appeared in September 2006)
It takes a little work to figure out how to scale the pictures, because you have to figure out the size of some element of the spacecraft that you can scale it to. Even if you know the length of the solar panels, that usually doesn't help because the image of the spacecraft is usually from an oblique perspective so that the solar panels are foreshortened. Fortunately, nearly every spacecraft has a circular high-gain antenna, and they usually report the diameter of that antenna somewhere. So, no matter what perspective the view of the spacecraft is from, you can get a diameter for the antenna to use to scale the image to.