And the CITY is laid out as a SQUARE, and its length is as great as the width; and he measured the CITY with the rod, fifteen hundred miles; its LENGTH and WIDTH and HEIGHT are equal.
This cubic city reminds us of the inner sanctuary of Solomon's temple (1 Kings 6:14-20-22). It was 20 cubits on all three dimensions. It is appropriate for the new Jerusalem to be this shape too, for even as God filled the sanctuary of the Old Testament tabernacle (Exodus 25:1-8-9), He also fills the new Jerusalem (verses 11 and 3).
From crystallography, we know the cubic-structured molecule is one of the most stable and stackable structures in our current world. Our common table salt is one such cubic molecule, and in an interesting sense, this gives us a whole new way of looking at what Jesus said about salt (Matthew 5:13-16) or what a believer is like. Since God saw that what He had made in the current world was "very good" (Genesis 1:1-31-31), He apparently saw fit to continue the pattern, into the new Jerusalem, the CITY which contains the essence of believers (verse 2).
From a spiritual viewpoint, many of us have a difficult time accepting the real physical dimensions of our "transformed" bodies (Philippians 3:17-21-21). But do not forget that after Jesus was resurrected, He showed Himself to have real physical dimensions (Luke 24:36-43 and John 20:19-31). The CITY, the new Jerusalem, is a real dimensioned CITY.