It seems to me that height or area are not individually enough to claim the title. A 200' pole on top of a dinner plate isn't much use, and something the size of a football field with only a pin for a gnomon won't cut the mustard either!
If talking about man-made dials then perhaps the answer is the dial with the largest base area and having a gnomon which casts its shadow to the edge. However, mention was made of posible other measures of "largest" - hows about the one which, given sun, would tell the time for the most number of hours in a day. In that case one might look at making a dial with a perspex plate and gnomon extending both above and beneath it. Sited on top of a mountain, as soon as the sun rises you'd get a shadow on the bottom of the plate, then the normal span of hours on the top of the plate, followed by a bit more on the bottom again. Anyway, size isn't everything! What about the smallest dial? David Higgon 51°27' N 0°15' W
