II Peter 3:10 sure sounds like the heavens and the earth that are now will be completely dissolved by the fire of God and then recreated as a new heaven and new earth. Those who think it is just a purging of the earth’s surface and the cleansing of the heavens relate it to II Peter 3:6 with the perishing of the world in the flood. It is the verses in Isaiah that speak of a New Heaven and New Earth in context with the millennial reign that leads some to believe it is not a complete annihilation of the heaven and earth.
Supposing it is, God would have to preserve the Saints of all the ages perhaps in the third heaven, the heaven of heavens, until the New Heaven and New Earth are created (in the time it takes for God to speak).
We are not told the size of the new earth, but even if it were the same size, Isaiah 40:4 says “Every valley shall be exalted, and every mountain and hill shall be made low.” Add to that Revelation 21:1 “no more sea” (even though I believe this is more than the earthly seas) we can see the geography of the New Earth is completely different. Plenty of room for the New Jerusalem.