Here's a presentation slide I've been meaning to put together for a while. It shows the major moons of the eight planets, to scale with each other. Below is a labeled version. You can download one without text here, and if you scroll down I've got a couple of alternative layouts.
Note: it's been pointed out to me that the Callisto image is from Voyager, not Galileo. I'll fix the text on the image eventually. --ESL
Here are some alternative unlabeled versions:
For the next one, I used a different Iapetus image, because it's adjacent to the Uranian moons Oberon and Ariel for which we don't have good lower-phase (fuller) global views; putting in the higher-phase one I used above made it look too big. Ganymede looks a little small for the same reason, and also because Titan's atmosphere makes it look bigger than it actually is.