Okay, so I've been going through several different sources for inspiration on how to portray battles in this quest. This system is probably going to need a lot of ironing out, so please note any problems you spot and, if you can, give suggestions on how to fix it.
Battles are divided into Minor Battles, Major Battles, Decisive Battles, and Kessen ("final battle"). The biggest difference between them is how many troops make up a base unit.
In a Minor Battle, the base unit is a troop of up to 30 men. Minor Battles are small skirmishes and raids, and at most will only have a few hundred men on each side.
In a Major Battle, the base unit is a troop of up to 300 men. Major Battles are actual serious engagements and may have up to a few thousand men fighting on each side.
In a Decisive Battle, the base unit is a troop of up to 3,000 men. Decisive Battles are huge, do-or-die affairs that will make or break a clan, and can have tens of thousands fighting on each side.
In a Kessen, the base unit is a troop of up to 30,000 men. As you might expect, a Kessen is a battle that drastically alters the shape of Japan's political landscape no matter who wins, and each one is a huge milestone for the quest.
Now, what do I mean by "base unit?" Simply put, it's the maximum number of troops an independent commander can directly control.
For example, Nobunaga has 150 men with him at the Battle of Kira-Ohama. Since Kira-Ohama is a Minor Battle, the "base unit" is 30, meaning Nobunaga can only directly control 30 out of his 150 men. So what does he do? There are two solutions.
The first solution is to just split off units and put them under the command of an officer to act independently of Nobunaga's personal unit. Nobunaga is the Taisho ("General") and as such can still give them commands, of course, but since they're independent units, they carry out those orders using their own judgment, and the unit uses their own statistics rather than Nobunaga's.
The second solution is to appoint sub-commanders in your unit. Each sub-commander you appoint allows the commander he's placed under to control up to another base unit's worth of troops, and the resulting unit will act as one unit in battle.
Both solutions are useful, and in most battles both will be used. With the first solution, you can act in multiple places on the battlefield, flank opponents, etc. With the second, you can use numbers to your advantage and also use sub-commanders to shore up any potential weakness in a commander or simply give him access to more skills and tactics.
At the Battle of Kira-Ohama, Nobunaga has 150 men, 5 base unit's worth of troops. Let's assume that he uses both of the aforementioned solutions to parcel out control of his men, and his troops' breakdown is thus:
Oda Nobunaga (Hirate Masahide, Takigawa Kazumasu) - 90 men
Kuwayama Shigeharu - 30 men
Kanamori Nagachika - 30 men
So Nobunaga is directly controlling 90 men, with Masahide and Kazumasu as the sub-commanders that let him have that many in a single unit. Further, he's split off two units and given command of one each to Shigeharu and Nagachika, who are beholden to his orders but are otherwise acting on their own judgment and using their own stats, skills and tactics.
If you don't have enough officers to assign to your units, you'll have "placeholder officers" instead. These placeholders have bad stats and no skills or tactics, so even a mediocre officer is always preferable.
This is all very divorced from the reality of Japanese combat at the time in a million different ways, but that would be an absolute unholy nightmare to try to translate faithfully into a quest, so f*ck it. I'll just mention how it actually worked in an information post somewhere.
So that's how units are arrayed. But what happens when they actually meet each other in the field? First of all, the important statistics for a unit are as follows:
Troops - Essentially a health bar, since if this number dips too low the unit collapses into rout and is out of the fight. This is a rare occurrence, though, and most of the time Troops is important because a sizable disparity gives an advantage to the larger unit. Particularly large disparities in Troops even allows for special Tactics like "Encircle."
Morale - Most often, this is the health bar that makes or breaks a unit. With high Morale, a unit performs at its best, and as Morale gets progressively lower their performance slips, and there's a growing chance for the unit to rout. At Morale 0, a unit routs instantly. There are a lot of ways to modify Morale and the rate at which its gained or lost, from environmental effects to training to special Tactics and Skills.
Drill - A unit's Drill is the degree of training they've received in combat. Units with high Drill get bonuses to their troop statistics like ATK, and also lose Morale more slowly and have a lower chance to devolve into rout. A unit engaged with an opponent with an appreciably lower Drill statistic gets an advantage to its performance, which can potentially offset (or add to) the advantage created by a disparity in Troops.
Troop Class - This is the particular "kind" of troop that the unit is made up of. Not in the sense of what they're armed with, but in their social class and the method of recruitment. There are three basic types: Chugen, Ashigaru and Samurai. Chugen are the lowest quality, being conscripted peasant rabble. Ashigaru are something like a semi-professional militia in most domains. Samurai are, of course, the elite and professional warriors. A unit's Troop Class determines both their minimum and maximum Morale and Drill, and also affects the type of armaments they can be equipped with. A unit can be "upgraded" into the class above them, but this can be expensive.
Troop Type: Mostly, this comes down to what weapons your unit is equipped with. Spears, Bows, Guns, etc. It also denotes whether they're mounted or not. Different Troop Types have advantages over each other and different base statistics.
ATK: The offensive power of a unit. Among other things, a commander's VAL stat will affect their ATK.
DEF: The defensive power of a unit. Among other things, a commander's LEA stat will affect their DEF.
SPD: The ability of a unit to cover ground. This is mostly for when units want to reach an objective first, or are trying to flee or to create space/close the gap with an enemy.
RNG: A unit's ranged capability. This is almost entirely a function of Troop Type, though very high Drill can extend a ranged unit's RNG. This is mostly to decide how long a unit can bombard an enemy who can't respond with ranged attacks, and how easy it is to keep their distance.