John Gill Commentary Revelation 21:15

"And he that spake with me had for a measure a golden reed to measure the city, and the gates thereof, and the wall thereof." — Revelation 21:15 (ASV)

And he that talked with me
One of the seven angels, (Revelation 21:9) for not a man is the measurer of this city, as in (Ezekiel 40:3) nor John, as of the temple, (Revelation 11:1) but an angel, the same that showed to John this great sight:

had a golden reed ;
not a common measuring reed, or rod, as in (Ezekiel 40:4) (Revelation 11:1) but one covered with gold, denoting the present glorious state of the church, being without spot or wrinkle, or any such thing:

to measure the city ;
the new Jerusalem, its length and breadth; see (Zechariah 2:1Zechariah 2:2) to show how exactly conformable this church state will be to the rule of God's word, even to perfection: and the gates thereof; to signify who had a right to enter into it, and who not:

and the wall thereof ;
its height, it being exceeding high.