I mean.. i imagined him as a colossal being with a mouth large enough to get around the throat of the world. He'd be gold or silver too and would only apear for the last mission (obviously, considering his size) That black dragon is just too evil looking and small. How does such a tiny thing eat a world?
edit: and there was a book to go along with me too. It involved mehrunes and lorkhan
He's just a dragon, in the same way as Akatosh is a dragon; there's no reason for him to be incredibly big, he's just the strongest of all Akatosh's children, and unable to be beaten by conventional means for some unexplained reason.