if you wander onto the miniatures page (TMP) remember to wear the shark suit as its not called PMT! for nothing & its best to search rather than ask as you may attract much mockery for not already knowing the answer to the question your asking.
Not much of a Napoleonic gamer myself, stoped in the 1980's but I have heard good things about Le Feu Sacre by TooFatLardies (TFL) which would work better I think for 15mm as its very much a big battle game, several divisions per side, for more details on that try asking on the Yahoo group LFS list or the TFL group.
I have also heard good things about Lassalle though I have not played it. I would also note I have heard good things about Republic to Empire (R2E), published by Barry of this parish but have not seen played it or studied it in detail. I am sure some one here will fill you in with more detail on
Finally there is Sharpe Practice by TFL which despite its apparent lightness of content, works well as a straight wargame for every thing between a company a side at one to one to several battalions a side at a figure to man ratio of 1:10 or 20 and looks splendid in 25/28mm. I mention it as I am adapting it to run actions in the late 1700's & early 1800's using mostly Warfare Miniatures for the forces.
I hope that helps
Ginger