I am not aware of a source/guide about unit scale, either on the forum or un the manual.
Not sure there is one strict scaling. The figurines are however difficult to count precisely.
In Table Top wargames, one model (figurine) represents a certain number of men. This is the scaling.
In FoGII: Medieval, the general case is, according to the manual : "one infantry or cavalry model on the battlefield represents 60 men, in 4 ranks, so a standard non-light infantry unit of 8 models in two ranks represents 480 men in 8 ranks".