It's the 20th size there was:
http://camera-wiki.org/wiki/Film_sizes_and_designations
'6x9' has always annoyed me too, because the picture size isn't 6 cm; it's accurately 2¼ inches. I'm not an obsessive fan of imperial measures, but if inches conveniently express the size, why not use them? Everyone says 6x9 though, and I don't have a tank, so I bite my lip and join in.