The maps in Ascoli's Day of Battle cover an area of (roughly) 9 by 6.5 miles. The short dimension is the more constrained. Converting to metric, this gives a minimum of about 86m per table centimeter. One hundred meters per centimeter gives a nice, round number on the safe side.
After a few years of experiment I have found that the smallest base size that works easily for me is 30mm square. I could end up with larger bases, but 30mm on the smallest dimension will be a basic guide.
Day length is interesting. It is easy for a North American to forget how far north European cities are. While the action proper was about 12 hours, a dawn-to-dusk struggle could easily run to 15-16 hours.
Focusing on 12 hours, however, with a six hour duration objective, we have to be able to resolve 2 hours of action each hour. At the peak, something like 20 division(ish) French units were involved. For the "Gravelotte-St Privat" battle the French Numbers are similar while the Germans have something like 15 larger and more complex divisions. At 20 "command units" a side, we would get something like:
- If a turn represents one hour, it must last 30 real minutes. Assuming movement is not simultaneous, time is shared evenly, and a two player game, that gives 15 minutes a side or 45 seconds a division.
- If the turn represents two hours, we have a minute and a half per divisions.
- 4 infantry regiments (in two brigades)
- 1 double battery
- for the Prussians (with small variation for its allies) a second double battery and a cavalry regiment.
On the other hand a slow player with a chess timer could simulate Bazaine very well indeed.
Of course, a formation could be a deal simpler -- one stand per division, one per brigade, guns factored in or out. But that would be another post.