Hi Baz,
If you have a look at the panoramic camera offerings from Linhof/Horseman etc the most popular aspect ratio for a panorama is 2:1 and roughly 3:1 which corresponds to their 6x12cm and 6x17cm 120 film cameras. Fotoman used to make a 6x24? which is a ratio of 4:1. But i find this to be 'too long' i generally stick to about 3:1
Ben