I have always viewed the 'units' in game as abstracts. For one thing, to be even vaguely 'historical' they would have to vary by Era/period dramatically.
For example:
The largest regular unit in ancient Egypt was the Pedjet of 1000 - 1250 men or 250 chariots.
The largest regular unit in Mesopotamia (Sumer, Akkad) was the Sanga of 400 - 700 men
In other words, about the size of a single Battalion in an army of the Industrial, Modern or later Eras.
Both had larger military forces, the Mesha in Egypt and the Erin in Sumer, but they simply consisted of "everybody available" and frequently translate as "Host" or "Corps".
There are no regular units in Classical Greece. All the phalanxes of the various city-states simply consisted of everybody who showed up with spear, shield and armor. The maximum numbers given for any single city-state are around 10,000 - the hoplites of Athens OR Sparta.
The Macedonian Army of Phillip II and Alexander and his Successors was a professional army with a regular organization. The Pezhetairoi ('Phalanx" of pikes) were in Taxeis of 1600 - 2000 men each, the Hetairoi or 'Companion Cavalry' in squadrons of 200 - 300 men (the Agema or Spearhead squadron was larger). Many of the allied parts of the army and light troops appear to have been in decimal organization of 10 - 100 - 1000, and the later Companion Cavalry and Hypaspists were in Chiliarchai of 1000 men each.
The Roman Legion is a case study in frustration. The 'classic' Roman Republican Legion was 50 Centuries. Since a Century means 100, that would be 5000 men, the figure usually quoted. Except that in the late Republic a Century was 80 men (the 'squad' or Contruba was 8 men, but it was led by a Decurion, which translates as "Leader of 10") In addition, the 'standard' Republican army (led by one of the Consuls) was 2 Legions plus 2 Allied (Italian) Legions, making the 'Legion Slice" closer to 8 - 10,000 men.
BUT during the mid and late Empire, the part of the Legion that could actually be deployed in the a field army was as little as 1200 men.
Virtually all of the Central Asia 'nomadic' Factions/Civs had a decimal 'organization' of 10 man groups, 100 man companies (a Cossack Company in the Russian Imperial Army and Soviet Army as late as the 20th century was still called a sotnia = "100"), 1000 and 10,000 man 'Units'.
Chinese armies varied even more wildly, since the 'military system' of almost each Dynasty was different. One notable feature, though, is the number of 'non-combatants' in each army - true of almost every army in history, but some Chinese accounts actually list them for a change. So, we have a figure for one Tang Dynasty Army of 80,000 men, but that total included over 50,000 'carriers' - civilians carrying supplies, tending horses, servants, etc - and less than 30,000 'troops' and most of the fighting was done by a single force of 1000 heavy cavalry!
In Medieval Europe the only 'permanent' military organizations are the High Medieval 'Lance' - which consisted of a knight and, depending on the kingdom and time, a variable number of 'assistants' including squires and other armored horsemen, light horsemen, archers, and the usual rarely-counted collection of servants. At the other extreme, for combat a Medieval European army was divided into 3 or 4 'Battles' with no fixed organization at all.
Post-Medieval Early Modern Armies started to be more uniform in their organization. While the Ottomans stuck to the 'traditional' Decimal organization, in Europe various 'standard' units became common: pikes in 'decimal' units of about 1000 men, then muskets, crossbows, halbards, and other 'auxiliary' weapons started getting added. The earliest fixed Pike and Shot unit recorded is the Spanish Colunela, or 'column' of about 1000 men, including 1/3 auxiliary crossbowmen, swordsmen or arquebussiers and 2/3 pikes. Three of these were soon combined into the famous Tercio. In the Netherlands the Medieval 'Battle' became the fixed Battalion with 1/2 to 1/4 pikes and the rest arquebuses or muskets, totalling 600 - 1000 men.
In most armies the standard military unit became the Regiment which, on the actual day of battle, could be divided into Battalions. It took about 150 years for the number be standardized, though: as late as the early 18th century the number of battalions in an Austrian Regiment depended on how many senior officers were available on the given day, resulting in 'battalions' from 400 to 2400 men in size!
By the Industrial Era in game terms (1740 on) the 'Modern' organization was standard in Europe and was eventually adopted throughout the world by the 20th century: a battalion of 400 - 1200 men (average about 800 men) combined into Regiments or Brigades of 3000 to 6000 men depending on the number and amount of supporting units, divisions of 10,000 to 25,000 consisting or several brigades or regiments plus an increasing host of supporting elements: artillery, antiaircraft, antitank, combat engineer, reconnaissance, tank, medical, supply, maintenance, etc. The division was, from the Napoleonic Wars until after WWII, considered the 'standard' and smallest unit that could operate independently for any length of time.
The in-game 'Corps' and Army' units are artificial. In reality, except in the Soviet Army in WWII (tank and mechanized Corps had standard organizations), neither was or is a standardized unit. A Corps was simply a variable number of brigades or divisions and the Corps HQ was invented to make it easier for the Army Commander to control large forces over wide areas (during the Napoleonic Wars). The Army was a HQ with a specific task and given whatever amount and combination of regiments, battalions, brigades, divisions and corps were considered necessary to accomplish the task. A modern 'Army Command' in fact, may be a territorial command simply controlling all the forces in a given area whatever they are doing, while the actual combat forces have tended to get larger with increasing numbers of 'support' troops - modern US Army Brigades are bigger than some WWII Divisions and cover correspondingly large areas compared to their ancestors in previous 'Eras'.
In game terms, we have a very few units, usually Uniques, that can be identified with specific historical units and numbers of men: the Garde Imperiale is graphically the Grenadiers a Pied of Napoleon's Guard, which were 2 regiments of 4 battalions, or about 3500 men total. The 'Rough Riders' were a single regiment of US Cavalry with an authorized total of just over 1200 men - which total they never took into combat. Graphically the 'Roman Legion' in the game is a late Republican Legion(airy), which would represent a Legion of 4 - 5000 men (plus auxiliaries that could easily number another 4000 + )
The Persian Immortals were a single group of 10,000 according to Herdotus, but the largest number recorded in a Persian document in battle were 2000 of them at Gaugamela against Alexander (which, admittedly, may be all that were left after almost 20 years of Civil War in Persia - Alexander was attacking a very badly battered Persian Empire that had been fighting itself for a generation)
If pressed, I would assume that each Era has a different 'size' of Unit, based on historical possibilities - early states simply did not have the resources or administrative structure to mobilize the numbers of men and horses and weapons that later states could, and keep them fed long enough to get them into battle.
So, I would put the average Unit at:
Ancient Era - 500 men: a Pedjet or equivalent.
Classical - 1000 men: the 'classic' Decimal organization
Medieval - 1000 men: No advance over Classical
Renaissance - 2500 men: A Tercio, Swedish or Netherlands Brigade
Industrial - 5000 men: an Infantry Brigade of 4 - 6 battalions or Cavalry Division of 4 or more Regiments
Modern and later Eras - 10,000 men: a Brigade or the fighting component of a Division
