A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
eze 42:2CHAMBERS OF THE PRIESTS: MEASUREMENTS OF THE TEMPLE. (Eze. 42:1-20)
Before the length of an hundred cubits--that is before "the separate place," which was that length (Eze 41:13). He had before spoken of chambers for the officiating priests on the north and south gates of the inner court (Eze 40:44-46). He now returns to take a more exact view of them.
eze 42:5shorter--that is, the building became narrower as it rose in height. The chambers were many: so "in My Father's house are many mansions" (Joh 14:2); and besides these there was much "room" still left (compare Luk 14:22). The chambers, though private, were near the temple. Prayer in our chambers is to prepare us for public devotions, and to help us in improving them.
eze 42:16five hundred reeds--the Septuagint substitutes "cubits" for "reeds," to escape the immense compass assigned to the whole, namely, a square of five hundred rods or three thousand cubits (two feet each; Eze 40:5), in all a square of one and one-seventh miles, that is, more than all ancient Jerusalem; also, there is much space thus left unappropriated. FAIRBAIRN rightly supports English Version, which agrees with the Hebrew. The vast extent is another feature marking the ideal character of the temple. It symbolizes the great enlargement of the kingdom of God, when Jehovah-Messiah shall reign at Jerusalem, and from thence to the ends of the earth (Isa 2:2-4; Jer 3:17; Rom 11:12, Rom 11:15).
eze 42:20wall . . . separation between . . . sanctuary and . . . profane--No longer shall the wall of partition be to separate the Jew and the Gentile (Eph 2:14), but to separate the sacred from the profane. The lowness of it renders it unfit for the purpose of defense (the object of the wall, Rev 21:12). But its square form (as in the city, Rev 21:16) is the emblem of the kingdom that cannot be shaken (Heb 12:28), resting on prophets and apostles, Jesus Christ being the chief corner-stone.
Everything was now ready for His reception. As the Shekinah glory was the peculiar distinction of the old temple, so it was to be in the new in a degree as much more transcendent as the proportions of the new exceeded those of the old. The fact that the Shekinah glory was not in the second temple proves that it cannot be that temple which is meant in the prophecy.