Al, can you give some dimensions for this sundial as I don't think it is the largest in the world. The great Samrat Yantra on Jaipur has a base of 44 meters and rises to a height of 27 meters. It's flanked by two large quadrants of 15 meters radius. It's combined with the Sasthamsa Yantra for measuring the zenith distance and declination of an object. We were there several years ago and it's amazing in its scale. Have photos if you want to see as there is a whole observatory complex there which is really amazing.
See
http://en.wikipedia.org/wiki/Jantar_Mantar_(Jaipur))
Allan