When creating a dragon, I’m not interested in hearing about the square-cube law.
However, even I must admit that this post by James Nicholl at tor.com is interesting: Light sales in science and fiction.
Light bouncing off a mirrored surface does not exert much force. A light sail one square kilometre in area, located at 1 AU, would experience about 8 newtons of force from the sunlight bouncing off it. 8 newtons is about the force two blocks of butter would exert on your hand as you held them up against gravity. Still, small forces for very long times can provide surprisingly large delta-vs. This eight newtons/kilometre squared is free and available for as long as the sun shines. Wikipedia is kind enough to provide some idea of the potential this offers:
I’m a sucker for tables. This one is pretty snazzy. Mars really isn’t that far away considering it used to take up to 100 days to sail from Europe to America (if the weather was awful). Look how low the tonnage is, though! You’re not moving much of a ship with those sails.
Nicholl provides plenty of examples of SF using light sails. This anime example sounds like fun:
…in Yūichi Sasamoto’s Bodacious Space Pirates … Plucky schoolgirl-turned-privateer Marika Kato and her crew of equally plucky schoolgirl space-yacht-club members set out on the Odette II, a light sail craft with a dubious history. They thought that history was safely buried, never to return; this being an adventure series, they are proved wrong.
I guess to me it seems possibly more sensible to use other forms of propulsion, with light sails to provide supplementary or long-term push. But I have to say, if I were writing science fiction, I would certainly set real physics aside and use whatever magic physics worked best for the story — exactly as I allow dragons to soar through the skies using magic and ignoring the square-cube law.