Unfortunately, it seems not possible at the moment. I tested this by modding the starting bases and colonies to have 40 troops each, and since when attacking invaders always use all their troops it seemed like a good test. During the first invasion mission, our side only had 24 troops attacking, and the defenders didn't have significantly more people.
So basically, the max amount of troops you can use looks like 24. I also didn't find any options to increase the troop cap, so I'm guessing battles against the other starting factions are intended to never be more than 20 versus 20.
Ysengrim
Coyote joined
Not much to say.