(Rev 21:16) And the city lieth foursquare, and the length [G3372] is as large as the breadth [G4114]: and he measured the city with the reed, twelve thousand furlongs. The length [G3372] and the breadth [G4114] and the height [G5311] of it are equal.

Below we include every New Testament reference (other than Rev. 21:16 listed above) where the same Greek words for length, breadth and height are used.

Length

G3372
me?kos
may'-kos
Probably akin to G3173; length (literally or figuratively): - length.

(Eph 3:18) May be able to comprehend with all saints what is the breadth, and length, [G3372] and depth, and height;

Breadth

G4114
platos
plat'-os
From G4116; width: - breadth.

(Eph 3:18) May be able to comprehend with all saints what is the breadth, [G4114] and length, and depth, and height;

(Rev 20:9) And they went up on the breadth [G4114] of the earth, and compassed the camp of the saints about, and the beloved city: and fire came down from God out of heaven, and devoured them.

Height

G5311
hupsos
hoop'-sos
From a derivative of G5228; elevation, that is, (abstractly) altitude, (specifically) the sky, or (figuratively) dignity: - be exalted, height, (on) high.

(Luk 1:78) Through the tender mercy of our God; whereby the dayspring from on high [G5311] hath visited us,

(Luk 24:49) And, behold, I send the promise of my Father upon you: but tarry ye in the city of Jerusalem, until ye be endued with power from on high. [G5311]

(Eph 3:18) May be able to comprehend with all saints what is the breadth, and length, and depth, and height; [G5311]

(Eph 4:8) Wherefore he saith, When he ascended up on high, [G5311] he led captivity captive, and gave gifts unto men.

(Jas 1:9) Let the brother of low degree rejoice in that he is exalted: [G5311]

Observations

Length: As there is only one other text that uses this word (Eph. 3:18) we are unable to learn much else about length.

Breadth: What we learn about this term is found in Rev. 20:9 where the camp of the saints is surrounded during the 'little season' by the opponents of God. It would seem that breadth here refers to the entirety of the symbolic earth (organized society under religious restraint) at that time.

Height: From the above references we can easily determine that 'height' as used here indicates to attain to something very high, i.e., something related to the Divine.

The text that seems to be the most direct for us is Eph. 3:18 which adds a fourth dimension, i.e., depth. This not only covers the other three but (by adding a fourth element) it seems to draw our attention to the fact that this city 'lieth foursquare.' Four indicates 'from every direction.' It also represents God's attributes of Justice, Wisdom, Love and Power.

Since the environment we live in is a three dimensional one of Length, Breadth and Height, by adding another (Depth) Paul is reminding us that our privileges in Christ not only encompass all of our daily physical lives but also our spiritual life as well. This last dimension (depth) refers to our spiritual life, without which we would certainly not be able to recognize nor appreciate the true beauty of the Divine Plan and our part in it.

This all suggests that this symbolic description of the city of New Jerusalem indicates it is complete, fully protected by all the righteous powers of the universe, as are all those who remain within this city.