Well that seems kind of dumb. If you have the option of placing any number of your men as militia, why not let you be able to do that at any unit size? I can't support my number of troops for the next 3 weeks or so it will take me to get an estate, but could support 20 troops for that long. I was counting on being able to drop them down to this number at any point.
Maybe this was an ingame mistake of mine, but still, limiting this to a certain number seems kind of weird to me.
