For damage :In Battlescape\TileEngine.cpp, function TileEngine::hit()
else if (part == V_UNIT)
{
https:// power 0 - 200%
const int rndPower = RNG::generate(0, power*2);
Stat adjustments based on difficulty : In SaveGame/BattleUnit.cpp, function BattleUnit::adjustStats()
Bigger base seems a bad idea, since it involves ugly scrollbars or redrawing all the base structures at a smaller size.