What of the 1:20 size? Works well for my Seven Years War mod.
And if a unit by itself is too small, you can merge it with others similar, which would free up slots.
So a regiment of cavalry in the Hellenistic period would be 50 men on huge; a Roman legion would total 240 men.
I admit this would make units small on Huge compared to other mods, but you get the advantage that you can accurately represent the organization, strength, and size of most ancient armies.
here's one scheme, for example (consular army, four legions):
2x velites (90 men each)
2x hastati (180 men each)
2x principes (180 men each)
2x triarii (90 men each)
2x equites (30 men each)
here I merge two legions' worth of men into one unit, for the sake of having sufficient strength, but also to allow the representation of the large auxiliary corps.
The other ten units would then have 1 general, and 9 auxiliaries of your choosing.
A late republican Legion could then be as follows:
1 "first cohort" unit (120 men)
1 "regular" unit (120 men)
You could also go with a single legion, 240 men strong, with eagle included.
A taxis of cavalry in this scheme would be 50 men; the phalangites can be made to represent 4 regiments (total strength ~240 men).