Well, looking at the sizes, Manhattan is about twenty-three square miles, making, on average, the X- and Y- axes a little under five miles each. Judging by the sprite, it's twice as long as it is wide, making the length about seven and two-thirds long and two and one-third wide, and, also judging by the sprite, is about at tall as it is wide at its highest point. The space it would require to fit in any one place would be a little over forty-one-and-a-half cubic miles. This, of course, is judging by xcalibur's thoughts on the subject. Compared to a mile-long Star Destroyer from Star Wars, the Black Omen is FUCKING GIGANTIC, which actually does it justice, if you ask me. Given the amount of weaponry that a Star Destroyer has, it would take at least a small fleet of them to cause significant damage to the thing. Very fitting for something so unique and powerful.
The more that I think about it, having the Black Omen in the Star Wars universe seems like a very dangerous idea, mostly with what could be done with it. If you were to put this all into d20 scale, like how a lot of other things are developed into the system, comparison of the infiltration and destruction of the Black Omen to that of a Star Destroyer would be like comparing a level twenty dungeon to conquering a layer or two of The Abyss.