for vista and win7 C:\Users\<<UserName>>\AppData\Roaming\Mount&Blade Warband
for XP C:\Documents and Settings\AppData\Roaming\Mount&Blade Warband
edit rgl_config.txt in Notpad
battle_size = 1.0000
change this to any number you want
battle_size = 6.0000 is what I'm running at, it'll give you around 600-700 men per battle and looks quite epic in Warband, surprised the engine can actually handle it. Though turn off Bodies and Ragdolls because the engine doesn't like them at this level, but all the other settings should be fine at this level. There is some bugs in Warband atm though that cause crashes on winning a battle or when ever the AI makes a successful retreat so be mindful of this.
You probably won't need the battle size changer mod ever again due to how easy it is to change this setting now.