It's the diameter of a city. A big city. So it's not a moon by any stretch of the exaggeration.

Also I don't think a moon is defined by it's size...

I didn't say anything about a moon, but until we know what it's made of, apparent size is not directly proportional to mass. That said, the average diameter of Deimos (one of Mars' moons) is 12.4 km, which (approximated as a sphere) gives us a surface area of 483 km^2. There are 198 cities in the world with more surface area than that, so yes, it could be called a moon.

What about The Death Star? Would that affect us?

Well that's the size of the moon... so I'd imagine as much.

Also how do you know it's the diameter of a big city? Are you judging it by the picture or did they say how big it was?