According to the film's SFX Supervisor, ILM's Ben Snow, Sokovia city was approximately 2km wide.
To start with, the city does not exist. “We filmed in several locations in Italy and the UK to cover the ground portions that happened in the street, so that gave us a lot of raw material for the design. But it ended up being one of our largest digital matte painting creations. that we had to build as a digital matte painting, plus the ground that it is sitting on top of, plus the engines that are driving it, complete with the pipes and the infrastructure under the ground with bits of earth falling off it. To get it right we studied landslide footage, building demolition and earthquake footage.”
“So, we had to not only build it as digital matte asset but also lift it out of the ground with the edge of it breaking away and the buildings crumbling like they were in an earthquake. It became a very elaborate process. Our traditional pipeline had been aimed more at single building destruction, like at the end of the Transformers movie. Here we were destroying whole sections of the city. So, we basically built on our pipeline so we could turn a digital matte asset into a generalist asset made in 3DS Max, then we took it through our rigid simulation system and sent it back for rendering in the generalist pipeline.”
For simplicity’s sake, let’s guess that Sokovia is at sea level and that Ultron . When the city is lifted, it looks nearly like a half-sphere of Earth. That half-sphere would be two cubic kilometers of dirt and rock.
And if all that soil and rock beneath the city has a density between that of soil and crust, then the total mass of Ultron’s makeshift asteroid would be around . That’s about the same mass as all the food the world produces each year.
In the film, Ultron is able to half-execute his plan by dropping the city (with the help of some thrusters we’ll ignore) at altitude before The Avengers vaporize it. According to a line Captain America delivers at about the same time the key on the lifting device is turned, “the air was getting a little thin.” If we guesstimate they were getting to a height where altitude sickness could set in, Sokovia was around 2,400 meters up when it started to fall.
A free-fall from this height (ignoring air resistance) would impart100,000 trillion Joules of energy to the surface where the rest of Sokovia sat. Because science communication dictates that I compare this to a nuclear blast, this amount of energy is about half that of the largest nuclear bomb ever tested, Tsar Bomba.
Thor was able to withstand 200,000 trillion Joules of energy when it exploded. While a 50-megaton impact is a gargantuan amount of energy, it’s not an extinction-level event. For example, humanity has witnessed 40 Ultron-level explosions of this magnitude since we've been around.