I was planning on setting up a spam army for when the Horde comes to force them into sending a massive army. But if size of your army doesn't necessarily mean they'll send in x times more men, then I might not bother. Regarding the 12,000 men they did send against you though, if the Unit size setting was on huge, that number would probably be double: 24000 or so, which sounds more impressive, even though you'd have more men too.