Charlemagne’s army

Kind of a tall order here.  It is not just military but social as well.

By the time of the Carolingians, feudal institutions were becoming better defined.  The land holding nobility (the knight if you will) held his enfoefment and benefice at the pleasure of the war lord (king or duke) through their agents the counts (sort of varsity nobility with higher favor of the king).  The principal obligation of the noble was military service (40 days on average) in exchange for exemption from taxation and assessments.

From the fifth century the 'knight' was a mounted warrior, with or without retainers, partially armored and utilizing pole arms and sword.  He was the ultimate weapon of his time in terms of hitting power and reach.  The most competitive warrior from 700 to about 1300 was the Islamic light horseman armed with sword and perhaps bow.  But no match in close combat with the Frankish-Germanic knight.  The knight was also known to fight with some determination and skill on foot!

In times of conflict, the knight might raise additional manpower from his retainers or tenants who possibly had some minimal skill in arms, but probably were more useful as laborers or craftsmen...saddlers, armorers, cooks and servants, etc.  As a remnant of late Roman practice, some may have been useful as crossbowmen, but socially it was becoming adviseable and preferable to NOT arm the noble's tenants.

I am not aware of the size of armies raised in Charlemagne's time, but they were most likely not large because of two factors:

First, taking manpower away from the estates of the nobles and counts negatively impacted the working and harvesting of the produce of those estates, and the wealth and well being of the nobles' families and peasant tenants.

Second, population around 800 AD was sparse and poor, and this would have affected the ability to feed both men and horses in any large number for any lengthy period.  Also, the campaigning season was ruled by the availability of fodder for horses and other animals...roughly May through October.  Once that was gone there was little to eat and no "middle class" regime to buy, store and broker anything that could be preserved.

In spite of the inflated claims and medieval accounts of historical events, I suspect the size of these "armies" was rather small.  The problem is that the reliable souce material is minimal and very subjective in nature. 

As far as effectiveness, they certainly were able to subdue and control the marches of the Carolingian-Frankish kingdom.  They must have been good enough.