I'm not sure who the smallest all time was.
The smallest corps I ever saw at DCI was the Royal Cadets from somewhere in Massachusetts. They had two percussion (a snare and a bass) and, I think, four horns ( two sops, two baris?) in 1997 division III prelims. They were the smallest corps I remember seeing.
The Marion Cadets marched five or six brass in both 1996 and 1997 but did not attend DCI in 1997. I think they usually had three or four guard and a percussion section that was big enough to make balancing with the horns a challenge! Those Marion groups were fun to watch and listen to!