Realistic mission-making is a topic I've thought about extensively, and the truth is that you simply can't always find the information you need about how military units are organized. As a solution, I categorize military units into a simple 4-tier system: maneuver units, tactical units, operational units, and strategic units. Please note that these are my own terms (and I have never served in the military), but they are based on real terminology that I've researched.
The goal is to categorize military units not by size, but by function:
-A "maneuver unit" is the minimum cohesive unit that typically deploys to the field to complete a mission; it is typically the lowest level unit led by an officer. It may travel in formation, or it may break up its formation into sub-units or even sub-sub units for tactical purposes. A platoon of tanks or infantry are a "maneuver unit," and a squad of infantry would be a "sub-unit" to the maneuver unit.
-A "tactical unit" is a grouping of maneuver units, led by a headquarters element, which is capable of performing military actions across both time and geographic space. Each maneuver unit belonging to a tactical unit can work in shifts with each other maneuver unit to accomplish the mission. For example, 1st Platoon could be patrolling, 2nd Platoon could be on standby as a "quick response force," and 3rd platoon could be resting. A "tactical unit" is basically a homogeneous grouping of maneuver units that can rotate between various duties and "down time" until the mission is accomplished. A company/troop/battery is typically a tactical unit, both for practical and historic purposes.
-An "operational unit" is a grouping of tactical units that includes a headquarters staff capable of mission planning (i.e. intelligence & recon) as well as providing combat or combat-service support (in CWA, that means MEDEVAC, refuel trucks, ammo trucks, and repair trucks). Operational units are typically less homogeneous than tactical units, but they are often still specialized (an infantry, armored, or combined arms battalion). A battalion is an example of an operational unit.
-Lastly, a "strategic unit" is a somewhat-mutable grouping of different types of operational units to deal with a variety of missions. Typically, a strategic unit deploys to an area of combat operations in its entirety (i.e. an entire brigade, regiment, or division deploys to the combat area in order to accomplish its nation's long-term strategic goals). Brigades/regiments or divisions are examples of strategic units, and they work hard to promote espirit de corps and pride of membership. Some strategic units have become very famous for valiant combat actions (like the 101st Airborne Division of the U.S. Army, for example).
When making realistic missions in CWA (or any other ArmA game), I typically try to set up operational-level units. In my fictitious scenarios, a "strategic unit" like a brigade/regiment or division has deployed to the theater of operations, but the particular island on which the mission takes place will host only some element of that strategic unit - in other words, one island will have one or two battalions of each side. ArmA games really can't simulate much more than that due to the limitations on the game engine as well as limits on physical space in the game maps.
On the island, I'll designate a forward operating base for each battalion and then deploy platoon-level units on patrol. I try to set up defensive systems that are somewhat autonomous so that if my deployed platoon detects the enemy (a "detected by" trigger covering the entire island), another platoon-level unit will automatically come to reinforce them. For the attacking side, I just set waypoints using the proper number of platoon-level units that would be available for an attack (assuming that some element of the attacking side will remain behind to protect its base).
To me, this approach represents a reasonable middle-ground between the extremely tedious process of researching the exact composition of historic Soviet & U.S. Army units on the one hand, or throwing a random but convenient grouping of military units together for arcade-style gameplay on the other.
I know this post was long, but I hope that my approach can at least help guide anyone wanting to design realistic scenarios.