Reed is μέτρον κάλαμον [metron kalamon] . Μέτρον [Metron] is an instrument for measuring.1 Κάλαμον [Kalamon] is the (dried) stalk of tall plants.2 The reed is a golden staff made for measurement. Ezekiel was shown the Millennial Temple and told to record the dimensions measured by an angel (Eze. Eze. 40:3-5). Previously, John was instructed to measure the Tribulation Temple (Rev. Rev. 11:1+). The New Jerusalem is measured to highlight its importance to God and indicate its real physical dimensions. Zechariah saw an angel who measured Jerusalem. The precise dimension were not given, but the measurement indicated that Gods attention was directed toward the city for enlargement and restoration (Zec. Zec. 2:1-5). See commentary on Revelation 21:2.
1 Frederick William Danker and Walter Bauer, A Greek-English Lexicon of the New Testament and Other Early Christian Literature (Chicago, IL: Uersity of Chicago Press, 2000), 515.