Good job Colin, apologies for not posting this info earlier but I've only just found the thread! I wouldn't worry too much about the length of the pelisse. I've read dozens of books dealing with this and I am no nearer a definative answer, I have always obtained the impression that the pelisse reduced in length towards the end of the empire and became, to some extent, a matter of fashion with the officers who could afford and get away with non regulation items. Rousselot states that, " these two articles (the Dolman and pelisse), were being shortened around 1808 - 1809" and " The barrel sash, therefore, was worn higher and the skirt of the Dolman extended only slightly below it".
I found these two illustrations in "La cavalerie Legere du Premier Empire" by Michel Petard . The diagram is presented as being taken from the 1812 regulations and shows the standard length of the pelisse as between 401 - 433mm, which is pretty short!
The photograph is from the Musee de L'armee Paris collection (8th Regiment) which does show a very
thin material in use, which makes me question the extent of it's practical use as an all-weather garment.
Rousselot also says that, (despite many models and illustrations suggesting otherwise), only the top five rows of the Pelisse cords could be fastened, being fashioned longer than the rest. I don't know where this comes from. It does not look that way in the above illustration although the photograph does look a little that way to me, in that the top five loops look marginally longer.