ModelScale (e.g. <ModelScale>0.5000</ModelScale>) in the unit xmls works (at least, it does for sovereigns and champions).

I haven't tried using UnitScale in the Race/RaceType xmls, but if that isn't working, I imagine you could modify ModelScale for the actual units (assuming you're copying from CoreUnits.xml to make your own).

Let us know if you get this to properly work. Everytime I've tried it clothing and armor gets messed up and sort of out of sync with the character model being either too large or too small. I've tried modifying the <EquipmentScale> or whatever and it doesn't seem to help either. Then again, maybe I'm not doing it right!

I'm also interested in unit scaling, 'cause i think that the units in tactical combat are too big, a scaling down will give more space for formations

I haven't gotten unit scaling working yet.