From the Wikipedia, http://en.wikipedia.org/wiki/Napoleon:
If Napoleon started out as a second lieutenant in 1785, and “rose through the ranks” after the revolution started, how could he have been only a corporal in 1796? A corporal is a noncom ranking higher than a private and lower than a sergeant, right? Or did the word mean something else on France in 1796?
(And BTW, why is Moammar Qaddafi still only colonel? Dude got no ambition! 
)
At the battle of Lodi, he took over the sighting of one of the cannons himself, which was a job usually done by a corporal.
He received a commission as a Second Lieutenant in the ancien regime army in 1785, three years prior to the initial events of the French Revolution. No doubt he resigned the commission when he returned to Corsica. His rank in the Republican army in 1796 was apparently only that of corporal.
Interestingly, there is a parallel of sorts in 1930s-40s Great Britain. A Brigadier Percy Hobart had the audacity to suggest in the pre-WWII British Army that massed armor attacks would change the future of warfare, and was dismissed from his command for doing so against the conventional wisdom of the top brass. He enlisted as a corporal in the Home Guard, from which Churchill recovered him and got him a general’s commission.
No. At the time of the Battle of Lodi, Napoleon’s rank was "General in Chief of the Army of Italy, and serving under him during the battle were Gens. Dallemange, Kilmaine, la Bonniniere, and Massena. He was the French commander on the field.
Hijacking the thread to Libya!
Why Is Qaddafi Still a Colonel? (from Slate magazine’s “Explainer” column.)
According to this BBC.co “Hitchhikers’ Guide” article, Bonaparte preferred to be closer to the action than most generals, and may indeed have loaded field guns himself, and so was given the nickname by his troops – he was never actually a corporal. The “little” part is indeed a figure of speech – proper conversion of French measure indicates that he was a perfectly average 5’6".
Thanx, Sergeant Amazing! 
IANA Napleonic historian, but I have read ‘War and Peace’ and suspect part of the reason for calling him a corporal was as a deliberate contrast to the Kings, Dukes, and hereditary pooh-bahs that made up the ancien regime and the leadership of Napoleonic France’s enemies.
And I can imagine both sides using it – for the revolutionary French soldier, calling him corporal was stressing the egalite of la revolution, while for the aristocracy of England, Austria, etc., calling him ‘corporal’ was stressing his low-class white-trash origins and behavior.
Fear Itself: If the preserved specimen is authentic, its size wouldn’t represent Napoleon’s endowment during life any more than a scrap of his preserved skin would indicate that his skin was dark-colored and leathery when he was alive. I think it gives people some satisfaction to think that Napoleon conquered Europe because he was short – that is, somehow less-than-manly – and ‘one inch long and resembling a grape’ adds a new dimension to that. Remember, though, there’s an anecdote about him breaking a bed, and lots of examples of Napoleon getting very angry. Whatever his stature and endowment, he certainly had sufficent testosterone.
Quercus: History has another character who the English called ‘corporal’ to stress his low-class origins and behavior; in Austria, though, they called him ‘Führer’. Napoleon’s title seems to have been mostly endearing, and I don’t think he considered it an insult. IIRC, another reason for it is that he took a hands-on approach to leadership – taking command of small groups, riding to the front, and so on. The sort of thing you’d call micromanagement if you didn’t like him.
I’ve heard that often and it puzzles me. Did no one at all in England know how to convert from French measure? Did no one at all in England ever chance to see the man in person? He’s always depicted as short in all of the literature and political cartoons I’ve read from the period.
I think you’re assuming way too much humility on Napoleon’s part. This was a man who at imperial coronation, took the crown out of the Pope’s hands and crowned himself because he didn’t want anyone to think his title of Emperor was lesser in precedence than the Pope’s.
Well, if someone says he’s five-two, one assumes that his foot and yours are the same. AFAIK (which isn’t all that far), it didn’t occur to the image-makers of the day – even those British who knew there was a difference probably found it more condign to perpetuate the inaccuracy.