well.... one way to change the units splash size is to go into the object editor, find the unit, scroll down to Combat, find combat - attack 1 (or 2) - area of effect (full,medium,small damage) and change the sizes.
As for the damage, about 7 spots below that you'll see combat - attack 1 (or 2) - damage factor - (medium or small). Changing that will change the percent of damage the spash does at each respective distance.
ex. Combat - attack 1 - damage factor - Medium 0.40 = %40