Marvin,
If you want to develop a scenario based upon a historical situation, start with what you know (likely the OB from whatever source you're gleaning your details). Look for a map (or maps) that closely aligns with the situation and use SSRs to ignore any terrain that might not make sense. Then determine what the protagonists (from your source material) are trying to accomplish (typically advancing or holding). It's typically true that the protagonists are not fully aware of the composition of the antagonists short of the obvious. They may know, for instance, that they were up against one or two MG positions or mortars, so add those. Then, scale the remainder of the antagonists to match the situation in order to make it interesting. Don't fully trust the protagonists ("we were being attacked by an entire division!"), as in the heat of battle they're probably not that trustworthy anyway. The advancing side should have 25% to 50% more units than the defender, depending upon how far they have to go and the nature of the terrain. Slap the units down and have at it! Adjust the OOBs and game length for balance after several tests. If one side or the other manages to squeak out a victory in the amount of time allowed, you should be good.
Have fun!