I likes me a big "S"! The pentagon it's in fits a broad chest quite well; the bigger it is, the more it accentuates Superman's physique. I always like to think that it should look a bit like it was bursting, or at least stretching.
On a live-action actor, an "S" that big
swallows up their chest and makes them look stupid. Dean Cain looked like a total moron with that gargantuan shield. The smaller one he wore in the pilot suited him much better.
Besides, it's stupid to have the emblem be that big because it negates the whole point of a blue tunic. What's the point of Superman wearing a blue shirt if the emblem covers his entire chest?