I see that I've managed to almost miss June without a post, so here goes!
Many years ago, I wrote a brief article for MWAN that proposed that most games that I'd observed as convention games, or games that I'd played in, or games that could be played in a reasonable amount of time followed:
a) THE RULE OF TWELVE: Simply put, any battle can be reduced to a 12 unit or so limit of the core type of unit in the army. Obviously, in all but the most horse centered culture, this will be 12 units of infantry. Artillery and cavalry would then be proportioned off of the 12 infantry units.
A second rule that makes for collection sanity, and games that look and play well is:
b) THE RULE OF REASON: Another simple rule - The total number of units in a game shouldn't be more than the square footage of your table. If your table is 5'x10', that would be a 50 unit total. That could be 25 units per side, or 30 vs. 20, etc.
The RULE OF REASON doesn't imply that games of this size are needed, or should even be a goal. It really is just a common sense rule - don't put too much stuff on the table!!! Games with too high of a unit density just turn into a version of a zombie hunt. Pick of the first unit - guess what? - there's another unit right behind it - eliminate it - and guess what? - there's another unit behind it.
Too many troops for the table size just leads to plodding games of attrition.
I should say that both the RULE OF TWELVE and THE RULE OF REASON are based on "normal" unit sizes of 12 to 24 infantry figures, not the "old school" figure intensive 40+ figures per unit. Yikes - I think units of that size require their own set of rules!