Hehe, well, there are some old products that would do that kind of stuff for you. Chainmail was the origins of D&D and provides a complete mass combat system. TSR published Battlesystem for 2e, which is pretty much a rewrite of Chainmail with a more specifically fantastical role playing bent to it.
Essentially both systems simply amalgamate some number of troops into a figure which operates kind of like a creature does in ordinary encounter combat. Each figure usually represents 10 identical creatures. Ground scale is larger, so a square represents 10 yards instead of 5 feet. Each turn represents the actions of the figures over say 1 minute of time instead of a few seconds.
Really large unique monsters and heroes or important NPCs are individual figures. Unique creature figures are always 1 square in size, but a really huge monster might take up a 2x2 area. Figures representing 10 large or bigger creatures, like a unit of ogres would also be 2x2.
Stats are pretty easy to generate for unit figures too. They have an AC and other defenses as normal for their creature type. Same thing for attacks where monsters with special powers can have figures with some extra attack or movement advantage or something like that. Movement rates can be the same as in normal combat.
Figure actions can be generalized from standard combat actions. So a figure can move, shift, or charge and basically gets a 'move action' and a 'standard action'. Cover, concealment, etc can also work as normal. Terrain can also work as per the normal rules essentially.
Whenever the focus of action is on characters interacting then you can work that using the mass combat rules if it is say a PC attacking a unit of orcs. If it is a PC fighting an NPC or unique monster then just go to the normal battle mat and resolve it using standard 4e combat rules. If the combat goes more than say 10 rounds then the larger battle moves forward a turn. In those cases I would recommend most ordinary figures would be 'minionized' to make them simple to deal with and let the PCs have a large effect on the battle.
The final stuff you need to deal with potentially are initiative, formations, morale, and command and control. In a pretty big battle figures can be designated to form units and move and attack together. Morale is probably best dealt with as just part of the unit's hit points. Things like being attacked with CA would simply do extra damage and if a unit is reduced below its hit point morale value it isn't necessarily killed, it just becomes ineffective and tries to leave the battle. Characters can make a skill check using diplomacy, insight, bluff, or intimidate to try to 'heal' the unit back up to a point where it can fight again. That would represent inspiring the troops, gathering up fleeing deserters, etc.
As for command and control I would basically have a commander, if it is a PC, specify a general plan of battle. The DM can provide the same for the enemy. Troops will follow the plan. In absence of direct PC leadership that is about all they will do. Elite troops may exercise more initiative, so they might for example automatically set themselves a secondary objective or modify their orders when it makes sense. PC commanders can change the orders of troops when they are nearby, so the character can take charge of a critical part of the battle. Designated NPC sub commanders might also give orders depending on their ability and motivation.
I have it on my RPGNow 'wish list' right now.
