Does size really matter?
Yes. Otherwise each side will sacrifice a unit of 5 infantry. Hell, one side could even involve a militia unit.
Time obviously does matter, though. A bonus could be done proportional to the size of one's army and fading with time. For example, if the fade limit is one month, and a realm had 10 000CS total, then having a battle with 5 000 CS would grant a +.5X bonus to recruitment (X being whatever factor deemed balanced), which would fade to being a mere +.25X bonus after two weeks, and +0X bonus after a month.
How do you deal with the fact that total CS changes over time? Sometimes radically, such as after a huge battle. Does this include militia, or only mobile? If it includes mobile only, then small realms under siege from larger ones are screwed, because they rely on large amounts of militia. If it includes militia, then just about everyone is screwed, as nearly every realm has significant militia forces somewhere.
But to get the full bonus (or avoid the penalty) in your proposal, you have to have your entire army constantly involved in battle. We already get enough complaints that war isn't exciting enough, and nothing but a continuous cycle of refit/smash/repeat. Doing something like this enforces, or even mandates, this undesirable cycle.
Any way you slice it, this type of proposed mechanic will really slow things down, and is sure to screw over someone who's not fighting the type of war you are spec'ing out with your proposal.
