‘And he who spoke with me had for a measure a golden reed to measure the city and its gates and its wall. And the city lies foursquare, and its length is as great as its breadth, and he measured the city with the reed, twelve thousand furlongs (stadia), and its length and breadth and height are equal. And he measured its wall, one hundred and forty four cubits according to the measure of man, that is of an angel.'
For the measuring reed compare Ezekiel 40:3 and Revelation 11:1. The fact that this is a golden reed connects it with the inner sanctuary where everything was made of gold. The dimensions of the city demonstrate its perfection, it is a perfect cube. In 1 Kings 6:20 we discover that the holy of holies in the Temple was also a perfect cube. This is God's new holy of holies. It is a perfect place. Thus the people of God as represented by this city are God's new Sanctuary (compare Revelation 3:12).
The dimensions based on an intensification of twelve confirm the connection with the twelve tribes of Israel and the twelve apostles. God's new sanctuary is His people. That this is symbolic and that not all is to be taken literally comes out in the measurement of the height. What is the height of a city, and how can it be a cube? It may mean its walls but this would be an unusual way of measuring the height of a city for it would have towers above the walls. It is rather an ‘ideal' description. The one hundred and forty four cubits of the wall is presumably its thickness. But this is not a brick-built city, it is the people of God, and what is being denoted is not size but perfection and quality and security.
That the wall is one hundred and forty four cubits (presumably its thickness) possibly combines twelve (foundations) with twelve (entrances) again stresses that the Old and New Testament people of God are seen together. Although it is the measure of man it is the angel who measures the wall as John quaintly explains.