Combat is much more tactical than in vanilla. This isn't vanilla CK2 where the biggest number wins. It matters a bunch, but if you depend on it then you're dead. You need to be careful, you need to weigh your odds, you need to consider terrain and leadership, and, above all, you need to scout. The AI has no fog of war, you do. Keep at least one small army of a couple hundred throwaways (the cities and godswoods are great for this) and check the major routes that you know the AI takes. Put them in places that will give you warning that an enemy is marching on you (and they are marching on you). Three provinces worth of warning is much better than having them appear in the adjacent province and you lose a few days from reaction time. If you see a doomstack you run. It doesn't matter how close you are to winning the Red Keep, you hightail it out of there if they outnumber you by as much as a hundred men. Giving up the siege willingly is much better than having the enemy smash your armies and kill or capture your commanders.
The small 200 men sellsword companies are great for scouting and they're dirt cheap for a Lord Paramount. Just be careful that recently raised levies don't form up and kill them off while you're not paying attention. If you don't use them to actively lift fog of war then keep them with the main army.
Above all, don't ever go into the Stormlands. The enemy will outmaneuver you every time and the only way to run is into a dead-end peninsula. The Boneway will never show mercy. If you want to raze the south while bringing more men down the Neck then stick to the Reach and the Prince's Pass.
