Looking for a quote by Napoleon (I think) vaguely paraphrased along
the lines of:

To be a great leader requires an empathetic understanding of man; but
to achieve leadership requires a politician to be power-hungry and
duplicitous; rare is a mans nature who can be so great and so small at
once.

I have been resisting this question because my attempts to find the
official French version that a translation would be based upon have
failed to this point.  However, since no one else is finding a
superior version thus far, I am happy to provide an answer that I hope
you will find useful.

I first became familiar with this quote in the film by Rob Lurie
called "The Contender", featuring Joan Allen, Jeff Bridges, and Gary
Oldman.  All references I can find to the quote are related to the
film.

Here is Napoleon's quote as it appears in the film.  (Frankly, I
prefer your paraphrased version of the quote).

"To get to power, you need to display absolute pettiness. To exercise
power you need to show true greatness. Such pettiness and such
greatness are rarely found in one human being."

References to this quote's appearance in the film can be found here:
Written By, "The Provocateur:  Rod Lurie could have been a contender
for Oliver Stone's title" by Joseph McBride, October 2000.
http://www.wga.org/WrittenBy/1000/lurie.html

Premiere.com "Strange Bedfellows" by Tom Roston, November 2000.
http://www.premiere.com/Premiere/Features/1100/1100_fea_contender2.html

"True Greatness" by Rob Lurie.
http://www.whysanity.net/monos/contender2.html

I have also provided a link to the Internet Movie Database for "The
Contender"
http://us.imdb.com/Title?0208874

Search Strategy:  First find reference to the quote in the film, then
try to find alternate or French versions of the quote.
Search Terms:  Napoleon pettiness contender

It would be wonderful to get the original french, or a more melifluous
translation to english, but that is certainly not your fault! Thank
you!

I think I found your quote:

"'A military leader must possess as much character as intellect. Men
who have a great deal of intellect and little character are the least
suited; they are like a ship whose masts are out of proportion to the
ballast; it is preferable to have much character and little intellect.
Those men whose intellect is mediocre and whose character is in
proportion are likely to succeed in their profession. The base must
equal the height.'"

Is part of a quote on this page:

http://www.napoleonseries.org/articles/napoleon/quotes.cfm

in the section "On Leadership and Men" about a quarter down the page.
That page even has the context of the quote as said by Napoleon.

Search Terms used:
"napoleon quotes" leader
://www.google.com/search?num=100&hl=en&lr=&ie=UTF-8&oe=utf-8&q=%22napoleon+quotes%22+leader&btnG=Google+Search