Missile attack minimum = 1 (at 0 the unit is displayed as out of ammo when in batlle--or at least it was in 1.1, so you had no missile attack.) Also, missile attack is converted to integers. If you enter 1.5 you get a 1. 0.5 = 0. -1 = 0. I determined these from play testing.

Minimum number of men = 6, but mounts can be lower for elephants at least. This can be used to reduce the size of elephant units. (For forest_elephants the number of crew must be double the number of elephants to go smaller...else RTW adds extra elephants when you test them on the field. e.g. 6,4 is the same as 6,6; but 8,4 gives reduced number of elephants by 1/3 and an extra crewman on each. Haven't tried 6,2 or 6,1 but the Oliphants work with only 2.)

Mass works up to 100 at least (Oliphant). I have used 99 before in my own tests. Haven't tried higher.