Actually, to correct Jim Noel, most of the O'Sullivan images are 10x12, which is still plenty big when you think about him hauling around hundreds of glass plates. The original images are wet plate collodion negatives on glass plates, printed on albumen paper. There was a wonderful exhibition of his work about 2 or 3 years ago here in Washington DC at the Museum of American Art entitled "Picturing The West". Many of the images featured in that article were in the exhibit. I know it traveled after it left DC, but I forget where else it went. In any case, the exhibition catalog, of the same title, should still be available on Amazon.
Thanks a lot for posting the link, very nice and interesting to watch.