We use toy soldiers, some of us have even tried 3 3/4" action figures, but it's been a long time since I've seen 11.5" GI Joe used in battle.
I remember a game at Fall-In! 2001 that used them to fight the Battle of Stalingrad. Giant tanks, even bigger cardboard buildings, and it took up a full corner of the All-Star Center in Gettysburg.
Of course, there are some problems with using the larger figures, especially when played with outside.