Base sizes are those from the Shako 2 rules as most of the clubs napoleonics are based that way. The ones I use are:
Infantry: 60mm x 50mm (3 bases for standard battalion, 6 figures per base)
Light Infantry/Rifles: 60mm x 25mm (6 bases for standard battalion, 3 figures per base. Allows depiction of skirmish order better I think)
Skirmisher Base: 90mm x 25mm (to represent a battalion's deployed light company, I use 4 figures per base)
Cavalry: 80mm x 60mm (3 bases for a standard unit, 3 figures per base)
Artillery: 100mm x 75mm (we have started using 2 bases per battery, 1 gun model plus crew per base)
Brigade Commander: Single mounted figure on a base.
Division Commander: 2 mounted figures on an Infantry base.
Corps Commander: 3 mounted figures on a base, don't have one myself yet will probably use a cavalry base.
We also use 2/3 distances when playing Black Powder and this particular game was on a 6 foot square table, which to be fair is the smallest we have used but then we are blessed with available space and terrain.
For Quick Reference sheets with different distances see here
perrysheroes.free.fr/spip.php?article259