The old style baking dish is bigger and smoother than the modern version and far more rare, so for conversion rate purposes I would suggest that 1 OG dish is equal or greater to 1 and a half of the inferior current version or can be substituted with 3 of the 9x9(?) squares which seem to have maintained some level of quality and are widely available.
My housemate who returned home to discover most surfaces in the house covered in pyrex pans full of gold liquid asked me why there were so many dishes all over the place and also: whatthehellldude? why do I have so many dishes full of
cat pee anyway?!