Diablo should be in the game. Assuming he is, he should hold the same mechanics and size (at least similar) to that of diablo 2 IMO. The Lord of Terror needs to look and feel terrifying, something which blizzard has yet to fail to deliver. Have faith.
i think in D1 blizz managed to make Big D terryfyin, i rembember that when you realse him the light around you begins to diminish and everything gets darker and darker... that was pretty scaring... i hope they come up with something similr in D3
How big will Big D get?