In Revelation 21:16 it talks about the New Jerusalem being approx. 1,400 miles square. How is the great multitude that no one can count mentioned in Revelation 7:9 going to fit into this city? I realize this is a strange question but it does puzzle me.
Revelation 21:16 describes how large the New Jerusalem, the city of God, will be in heaven.
The city is laid out as a square, and its length is as great as the width; and he measured the city with the rod, fifteen hundred miles; its length and width and height are equal. (NASB) Rev. 21:16
A quick calculation reveals that the city would be able to contain about 1,500,000,000,000,000 apartments if each apartment was 12 feet high (3.66 m) and contained 2800 sq. ft (260.1 sq. m). Now that is a lot of people. If we reduced the size of the apartments we would be able to put more people into the New Jerusalem. Unfortunately, we do not know how many people God will put into the heavenly city and do we not know the size of any rooms. That is the first half of our answer.
The second part of the answer is found in Rev. 7:9 where we read,
After these things I looked, and behold, a great multitude which no one could count, from every nation and all tribes and peoples and tongues, standing before the throne and before the Lamb, clothed in white robes, and palm branches were in their hands . . . (NASB) Rev. 7:9
You have asked a great question. The passage does say that no one can count the number of people who will live in the New Jerusalem. So, lets see if this is true. If we start by assuming that our numbers above were accurate, we could start counting the 1,500,000,000,000,000 people right now. How much time would be required to finish counting that many people? If we assume that someone was able to count 469 people a second and never stop counting, it would take that person 1,000,000 years to count the “great multitude.” That is, no one would live long enough to count all of them.
Your question was not strange. In fact, the answer to your question reveals the accuracy of the book that God wrote – the Bible. God does not lie (Heb. 6:18) and since the Bible was written by Him (2 Tim. 3:16) it is truthful. May the Lord bless you as you study His Word.