As I recall, the world's largest piece of glass was found in Iran. I think there was an article about it in Scientific American back in the 1950's. It was found in a qanat (underground aqueduct). It's rectangular, and it might be a door. I don't know why it was in a qanat. You can't see through it, because it's very clouded and highly colored.
I can't find any information about this piece of glass on the Internet, which is surprising. Searching on "world's largest piece of glass" gets lots of links to the mirror of the Hale telescope and a couple links to the younger ex-President Bush, who apparently is thinking about a project to make the next world's largest piece of glass.
Does anyone know of a link to the one found in Iran? I'd like to read what is known about it, and where it is currently located. And, of course, I'd like to see some pictures.