In 1975, Don Lowry released a set of rules called Grand Army. Designed for 9mm !? and 15mm figures, it allows players to recreate large scale battles for Napoleonics and the American Civil War. Grand Army now is available online.
En Avant is one of Jim Wallman’s “One Brain Cell” rules, this time for Napoleonic warfare. It allows you to conduct a brigade sized engagement in about 20 minutes using 25mm – 28mm figures. Just scroll down to find the links.
The Napnuts site has a set of rules for conducting a Napoleonic Peninsular campaign. The Peninsular Campaign makes for a good campaign because of its relatively limited scope.