not sure if there is a mod that only handles unit-stats to lenghten battles ..
do know there has been plenty of discusion about it and several mods have a balance that lenghtens battles compared to vanilla ..

I guess a couple basic ways to change unit-stats to lenghten battles without completelly having to rebalance units would be;
-change 1 HP to 2 HP, 2 HP to 3 HP .....
-give defensive stats an x% increase
-give offensive stats an x% decrease


G