During the invasion of Russia, XI CORPS under Augereau lists as part of the 30th Division:
Brigade: Breissand
6th Provisional Demi-brigade (1/0) (61/2,760) consisting of
The 4th battalion 16th Légère Regiment (21/646)
4/2lst Légère Regiment (14/721)
4/28th Légère Regiment (16/779)
Along with a "Provisional Battalion" consisting of
1 Company of 5/28th Line Regiment (2/148)
2 Companies 5/43rd Line Regiment (3/240)
1 Company 5/65th Line Regiment (4/226).
Now my reading of XI Corps was that it was the rag-tagandbobtail elements of the Army, including quality troops like the Neapolitans and Penal battalions along with smaller elements with no other obvious home like the Wurzburg contingent. We can assume from this that the French at least would combine small units to make a more useful formation
I assume that this campaign is set in the real world, Prussia, Austria, Russia, Spain France 1814? And a lot will depend upon whether the French are "winning" or getting a bit of a pasting. It will also depend upon whether a country is "friendly"? how close are they to the depots, will there be reinforcements?
Are the Governors of the local towns stirring up the populace against the French, (Spain) or are they seen as Liberators / defenders (Poland/Lithuania / France 1814).
The question for me would be, should the commanders seek to maintain "useful" sized units, better for the wargamer, or would the commander seek to maintain the esprit de Corps associated with the Battalion or the Regiment, perhaps more realistic. There is also the consideration of unit Morale, normally, at the start of the battle, the unit is at full strength and is at its normal morale level. If you are playing a campaign, then you may have to consider what Pierre and Jean-Paul think about what happened to their comrades? Would it be better to "accidentally" injure themselves, to slink off into the night?
Having read Quarrie's Napoleon's Campaigns, the description he gives of the French losses in Spain through desertion, sickness and guerrilla action are immense, while the losses, especially in terms of Horseflesh suffered by the Grande Armee as it entered Russia in 1812, due to lack of suitable fodder and Murat's Cavalry being kept in the saddle almost continuously are scary. It is also clear that on the retreat, only those units that maintained morale survived, those that broke up into smaller groups were picked off by the Cossacks or the local populace. Hence the so called "sacred" squadrons of Officers, the troopers having become dismounted or lost.
History tells of the number of failed invasions where the population has risen up; if you need to take a guard to go behind a bush, or a squadron of cavalry needs to accompany a messenger, things are probably not looking good. If the population feeds your troops, gives them shelter and is generally not trying to sew your eyelids together, you may keep morale high.
Once a unit on either side reaches or approaches XX% casualties, to be determined by the umpire, I would suggest that there is a chance that the unit will just evaporate, troops will wander off, join brigands, or just go home. The factors that affect this will be the basic morale of the troops, Guards will be less likely to evaporate than Militia. Troops who have won an easy victory will be less likely than those who have had a good kicking. However, winning is not the only measure, a win against a smaller foe, which would be expected, where the allies suffered proportionately higher casualties would not necessarily be an advantage. Are the units far from home, are they fighting to rid their country of an invader, do they really want to be there?
Is the phrase Liberté, Égalité, Fraternité, on their lips or are they fed up with fighting to make some Dude rich?
I look forward to reading what you decide.