Andy, larger units will allow them to engage in multiple attacks before requiring resupply. Units should be large enough so that you only need 10-20 of them rather than 100-200 that you need now. Having more expensive weapons will reduce the number of weapons that players have. When you combine larger units with fewer weapons in player's inventories, they work together to drastically reduce the insane amount of unit micromanagement that goes on today. Ammunition and maintenance should be very cheap so that players can maintain standing armies. There should be a benefit to maintaining a standing army, and a penalty for disbanding units. Every month a unit is deployed, it should gain 1 fighting level, to a maximum of 100. This could be called professionalism or training level. The hope is that when a unit has maximum training level, a player may choose to withdraw the unit, or wait for resupply, rather than disband and reform it to get maximum weapons back into it because disbanding and reforming would cause it to lose all of its training level. Hope that makes more sense!
Also, I just want to add that if we increase unit size but maintain the current 1000 weapon attack limit, the number of defenders participating in a battle will likely need to be limited as well. I think a larger limit will be needed if unit sizes are increased. Perhaps 5000 limit. Also, you could decrease the accuracy of all weapons proportionally so that the number of casualties are lower per attack. This will also make supply more important as ammo use will increase. As long as ammo is cheap, it won't increase the cost of war.