No one really knows how big heaven is, and the Bible doesn't give us a direct answer. But we can surmise that heaven is quite large.
How big is heaven?
First, we should clarify that there are a few different things meant by the word "heaven." We talk about heaven in the sense of where the stars reside, where God dwells, and where those who have put their faith in Jesus will dwell for eternity. Each of these is a large place.
When we look at the word "heaven" in the Bible we see a vastness that the writers could not explain. In Hebrew, the word shameh or shamayim refers to the sky or everything above the earth and all that is visible from it—beyond the atmosphere, stars, and all that can be seen. In Greek, ouranos means the sky or the place where God lives, or even "an eternal realm of happiness and glory." When the writers chose the word meaning sky or everything above the earth, they were saying that heaven is vast, lofty, and of God.
When you look literally at what they described, you can see that anything larger than what we can see is beyond comprehension. We have been able to see, with the Hubble Space Telescope, parts of the universe so far away it is hard to understand. The nearest galaxy to our own, Andromeda, would take us 2.2 million light years to reach—or 37,200 years at the rate of 18,000 miles per hour. Can you say warp speed, please?
When we look at the Bible, it shows us that God lives in heaven and He is infinite with no beginning and no end to His years (Psalm 102:27), there is no end to His reign (Luke 1:33), He is unchanging (Hebrews 1:12; James 1:17), and He is the Creator of earth and heaven (Genesis 1:1). Certainly His dwelling place is not small.
John, who was given a look at heaven, writes, "After this I looked, and behold, a great multitude that no one could number, from every nation, from all tribes and peoples and languages, standing before the throne and before the Lamb, clothed in white robes, with palm branches in their hands, and crying out with a loud voice, 'Salvation belongs to our God who sits on the throne, and to the Lamb!'" (Revelation 7:9–10). Though we don't know the measurements of heaven—where God and His followers who have died are—we do know it is big enough to fit a great multitude that cannot be numbered.
John is later given a vision of the new heavens and new earth, which is where believers will ultimately spend eternity. Out of the new heavens, a New Jerusalem will descend (Revelation 21:2). The New Jerusalem is only one part of the new earth, but the Bible does give its measurements as approximately 1,400 miles in each direction, possibly in a cube or pyramid shape. And that is just one city. So no doubt our eternal abode is quite large.
Is there really an afterlife?
Where do you go when you die?
Is heaven real?
Where is heaven? Does heaven have a specific location?
How can I be sure that when I die I will go to heaven?
Truth about Eternity