I used to play with large units, having decided huge was too much of a burden for the AI with the whole increased unit support costs and 2 turn training thing. But the last couple of campaigns, I opted for medium? or whatever the one before large is. You still get plenty of men on the battlefield but I find the lower support costs allow both you and the AI to churn out more units. It's also a bit easier to stick a variety of units into the keep's garrison, as on large and huge, a couple of spearmen unit tends to be all you can fit in there.