He is 6" total, but over two inches of that is whip. Without whips he is about 3.5 inches. Is yours all food crazy and active all day? Mine comes out every time I drop a pellet of wafer in there, and goes nuts on his zucchini until it's gone.
As for aggression, I've seen him slash at almost every fish in there. He doesn't go out of his way to beat them up, he just gets them out of his cave zone with a quick, mean attack. I have seen a few scratches on the other fish but nothing serious.