Following idea in reply #6, google can't find bigger than 20 mm LDR. Probably, solar panels would works as HUGE LDR sensor. It's even would be possible to assemble big linear array, stacking 10 panel in a line:
http://www.amazon.com/Charging-Extensions-ReStore-External-compatible/dp/B008I64E64/ref=lp_3236381_1_22?ie=UTF8&qid=1357354493&sr=1-22 