That is large:

 

Revelation 21:16 And the city lieth foursquare, and the length is as large as the breadth: and he measured the city with the reed, twelve thousand furlongs. The length and the breadth and the height of it are equal.

 

The land area of the city is (12,000 furlongs)², which is like ¾ of the current land area of the US.

However, if the city is cubic, rather than pyramidal, then volume of the city is

            (12,000 furlongs)³, which is like ten times the current volume of the oceans.

But if it is pyramidal, then the volume = ⅓AH = ⅓L²H = ⅓L³, which is still like 3⅓ times the current volume of the oceans.

 

Also, is the expression, “The length and the breadth and the height of it are equal”, similar to:

 

I John 5:7 For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.