What does Revelation 21:15 mean?

The angel who spoke to the apostle John had a gold measuring rod with which to measure the city, its gates, and walls. In Revelation 11:1 we read that John received a measuring rod and the command to measure the temple, the altar, and the worshipers. God is a God of order, and the fact that the angel in chapter 21 had a measuring rod for measuring the city, its gates, and walls shows that New Jerusalem is a literal city, not a mystical city. If it were a purely symbolic idea, or a metaphor only, it would be impossible to measure it.

Apparently, God who designed and built the city (Hebrews 11:10) wanted John and us to realize the city is big enough to accommodate the saints of all periods of history. The angel's measuring rod is not simply a staff like the one mentioned in Revelation 11:1; it is a rod of gold. Everything about New Jerusalem is glorious, including the rod the angel used to measure the city.