Basically, I see two main ways to do mass combat. It might be good to use both, the more involved when it involves the PCs and is more plot important, the other when not.
The more involved way would be to stat up the opposing forces, the exact details of what stats they might have would be decided later, but they would have pools analogous to those of a character, and combat between two forces would take probably 15 minutes to an hour or two.
The simpler method would be to just make opposed rolls for the forces, with each side being given bonuses for size, leadership skill, equipment, ect. Dice pools would possibly change depending upon the situation, a battle on an open field being less random, and a battle in an urban environment would be more random.