Jamieson, Fausset and Brown on Ezekiel 42:16

16. five 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 (Is 2:2-4; Jr 3:17; Ro 11:12, 15).