As a default take the average of total army size on both sides and reduce it by two. This is the amount of casualities taken by the winning side. The loosing side takes casualities as indicated by the win-loose ratio. The ratio can be translated to unit Size values by taking into account that the mancount triples for every Size-increase, ie. Anyway here are some kind of figures of what different sized armies are like.
Size is the size of the society.
Nominal is the nominal army size for that society and also shows how big is an army of size 'Fair'. During peace time, nations do not have armed secondary forces. Forces equal to one less than the specified size can be raised without problems. In case of severe losses in battle, these decreaments start to effect future years, too. In battle, an army without a secure supply line suffers a -1 to force for every year it is cut off.