The game should display smaller units just fine, as long as their height is at least... 6, I think. If it's less, then the origin point of the bullet is generated below the surface.
I know Otto Hartenstein tackled this problem with ultrasmall units, but I'm not sure if it was included in OXCE. Still, this does not really concern you, if you want to make the units only half as tall (which would be 11 for humans).