Gardner Dozois linked to this on a list we’re both on. It starts with our moon and then takes progressively bigger objects and sizes them against the previous, ending with the largest known star, VY Canis Majoris, a Red Hypergiant.
(Well worth clicking through to YouTube and Watching it in HD.)
VY Canis Majoris has a diameter of 2,800,000,000 kilometers. I did some very minor math: if it were in our solar system (instead of our own sun) it’s edge would extend out to the orbit of Saturn.
There are lots of videos out there comparing sizes, but this is the best one I’ve seen so far. If you’ve got a better link, do put it in the comments. No rickrolling. No porn.
(I take it back–if you want to rickroll me, go right ahead.)