"The heavenly messenger measured the holy city and it was twelve thousand furlongs or fifteen hundred miles: the length and the breadth and the height of it were equal, as we have pointed out — a cube. Some have supposed that a literal city must be meant since various measurements are given. However, we must always keep in mind that this city is the bride, the Lamb’s wife! Therefore such dimensions must necessarily have a symbolical and spiritual meaning in relation to the body of Christ! The measurements are in multiples of twelve which are a symbol and signature of God’s purposes in His elect. The number twelve often enters into the symbolism of the book of Revelation — twelve tribes, twelve stars, twelve apostles, twelve angels, twelve foundations, twelve gates. So here the number twelve is found in the measurements of the city: each side is twelve thousand furlongs and the wall is one hundred and forty-four cubits, which is twelve times twelve."
Much more in the link:
https://www.godfire.net/eby/Rev/rev246.html
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"Twelve is the number of divine government. One thousand is the number signifying the day of the Lord (II Pet. 3:8). Twelve times one thousand is God’s light and illumination (day) multiplied by God’s authority and rule (government) — the perfect and full dominion of God by the revelation of His glory without mixture, limitation, or darkness. The Holy of holies of God’s eternal kingdom is, therefore, in type, 528,000 times more glorious than the Holy of holies in the Mosaic tabernacle that was a cube of only ten cubits or fifteen feet each way! The true measure of the city is the Lord Jesus Christ: "…unto the measure of the stature of the fullness of Christ" (Eph. 4:13). The people who come to this measure will be the glory of God, the perfection of beauty, and the light of the world in the fullness of what that means, and will bring the total restoration that God, who cannot lie, has promised to all creation. Hallelujah!"