create a new mod folder (or use one you already like) in C:\Program Files\Stardock\TotalGaming\GalCiv2\DarkAvatar\Mods
copy the abilitybonuses.xml file from C:\Program Files\Stardock\TotalGaming\GalCiv2\DarkAvatar\Data\English into the new mod folder.
open the file in notepad, go to the end of the file, and create a new linebreak before the very last line, and paste this into it.
{Ability Name="Miniaturization"}
{AbilityIndex}29{/AbilityIndex}
{AvailableOptions}3{/AvailableOptions}
{Option0Text}Small Parts{/Option0Text}
{Option0Bonus}10{/Option0Bonus}
{Option0Cost}4{/Option0Cost}
{Option1Text}Tiny Parts{/Option1Text}
{Option1Bonus}20{/Option1Bonus}
{Option1Cost}6{/Option1Cost}
{BonusUnits}%{/BonusUnits}
{Option1Text}Microscopic Parts{/Option1Text}
{Option1Bonus}30{Option1Bonus}
{Option1Cost}7{/Option1Cost}
{BonusUnits}%{/BonusUnits}
{/Ability}
you'll have to replace these { } with less than and greater than signs. hopefully the forums will preserve the tabs - otherwise you'll have to eyeball it to match (though, i'm not actually sure they do anything except help humans organize the codes). balance the number values as you see fit.