What does Revelation 21:16 mean?
The dimensions given here for New Jerusalem are at once specific and yet vague. The description given here gives size, but not shape—so this might be describing a cube, or a pyramid. The city "lies foursquare", so its base is square. Its length and width are equal, each measuring 12,000 stadia or approximately 1,400 miles (2,250 km), about the distance from New York City to Dallas, Texas. The area of the base is about 1,960,000 square miles (more than 5 million square km).The city's height—either the tallest buildings or the top of some foundational structure—is also 12,000 stadia or about 1,400 miles (2,250 km). The question of whether the city is a cube or a pyramid is debatable. Typical architecture of John's era would have used a pyramid shape rather than a simple cube for something like a city. However, regardless of its shape, the presence of oceans, lakes, rivers, and streams on the millennial earth seems to suggest the earth could not accommodate such a large city. The new earth has no sea (Revelation 21:1), so some scholars suggest New Jerusalem hovers like a satellite city over the millennial earth but descends to the new earth (Revelation 21:10) after the millennium.
One thing is certain: New Jerusalem is large enough to accommodate all believers saved throughout all of history. The dimensions of New Jerusalem, whether cubic or pyramidal, are enough to provide ample room for billions upon billions of people. Whereas most ancient cities just grew without planning, New Jerusalem follows the plan of God, its Master Builder.