OK!! 12,000 furlongs on a side, breadth, width and height are equal- a cube. One furlong = 607 feet. X 12,000 furlongs = 7,284,000 feet = 1379.5 miles!!! That city, if placed on the earth, would stretch from San Francisco to St. Louis. If I'm not mistaken the moon is about 2160 miles in diameter, or about one fourth the diameter of the earth. So the length of that city would be about 65% of the diameter of the moon. Such a cube would be virtually unsupportable if placed on the earth.

However, it was seen descending out of heaven from God. So, what if, rather than actually coming to the surface of the earth is was placed as a near-earth orbiting satellite? That would solve all the gravitation problems. The inhabitants and all objects therein would be weightless!!! The inhabitants could occupy all three dimensions, rather than the two to which we are accustomed under gravity.

But why such a large building? In the Book of Moses 7:62-64 we read

And righteousness will I send down out of heaven; and truth will I send forth out of the earth, to bear testimony of mine Only Begotten; his resurrection from the dead; yea, and also the resurrection of all men; and righteousness and truth will I cause to sweep the earth as with a flood, to gather out mine elect from the four quarters of the earth, unto a place which I shall prepare, an Holy City, that my people may gird up their loins, and be looking forth for the time of my coming; for there shall be my tabernacle, and it shall be called Zion, a New Jerusalem. And the Lord said unto Enoch: Then shalt thou and all thy city meet them there, and we will receive them into our bosom, and they shall see us; and we will fall upon their necks, and they shall fall upon our necks, and we will kiss each other; And there shall be mine abode, and it shall be Zion, which shall come forth out of all the creations which I have made; and for the space of a thousand years the earth shall rest.

The Lord's creations are infinite in number. (See Moses 7:20), and there is no doubt that our Father's children living in mortality on other worlds know much more about us than we do about them. This earth is where their Savior was born into mortality, and worked out the great atonement. That atonement will bring to them, as well as to us, a universal resurrection; and his infinite suffering was sufficient to pay for all the sins committed by all the people living on all the worlds that our Father has created, if we but repent of our sins and follow the precepts taught by the Master.

We read in the Doctrine and Covenants, Section 88, where the Lord calls the planets, Kingdoms. He then states (D&C 88:68) that he visits all his kingdoms

And thus they all received the light of the countenance of their lord, every man in his hour, and in his time, and in his season-- Beginning at the first, and so on unto the last, and from the last unto the first, and from the first unto the last.

We also read in Doctrine and Covenants 38:11-12,

For all flesh is corrupted before me; and the powers of darkness prevail upon the earth, among the children of men, in the presence of all the hosts of heaven-- Which causeth silence to reign, and all eternity is pained...

So the New jerusalem will come down to this earth, which is destined to be the Savior's eternal home, and all the Zion people (those who live by every word that proceeds forth from the mouth of God) will congregate here to meet their Lord when he comes to initiate his great Millennian reign.

Since this is the most wicked world in the universe (see Moses 7:30) there must be many more Zion people on the other worlds than there are on this one. And a Zion city of the size described by John would probably be required to accommodate them all. How many people could such a place hold? Well, if you allotted 1000 cubic feet per person- a 10x10x10 room, it would come to about 10 to 15th power. Or, if the earth held 10 billion people, it would be about 100 million times the population of the earth. Talk about wonders in the heavens!!!