The C&S Model
Each sub-unit of the domain would be a fixed type. A large manor, a small manor, etc. Expanding the domain would involve saying “I want to establish a manor here”, with adequate money, time and workers, the manor would come into being, and from then on would provide fixed income and troops.
A small knights manor provides a Knight, Squire, 4 horsemen, 20 footmen (10 guard/10 commoners) for 40 days service a year and a certain income. If they get killed, the manor's heir needs to find substitutes/replacements by the next year.
DM's would have some basic maps of manor houses etc. The only thing players would need to know is whether they held the manor (or perhaps whether it had been ravaged, bringing in lower income).