Praise be to Allah, and may peace and blessings be upon the Messenger of Allah.
When a person dies, his body decomposes and perishes, except for the tailbone (ʿajb al-dhanab), which is a small bone at the bottom of the spine. The Prophet ﷺ informed us that when the Hour comes, Allah will send down rain from the sky, and people's bodies will grow from it just as plants grow. Then the creation will be reassembled and they will be resurrected.
The Prophet ﷺ said:
“Then Allah will send down water from the sky, and they will grow as vegetation grows. There is nothing of the human being that does not decay except one bone, which is the tailbone, and from it the creation will be reassembled on the Day of Resurrection.” Narrated by al-Bukhari and Muslim.
The scholars explained that the tailbone (ʿajb al-dhanab) is the part located at the bottom of the back, and it is from this remnant that the original creation remains for the body to be reassembled.
A person will be resurrected with his body, then gathered and held to account. After that, the characteristics of the people of Paradise will differ from those of the people of Hellfire.
Allah will enlarge the bodies of the disbelievers in Hellfire. It is authentically reported that the distance between the shoulders of a disbeliever is a three-day journey for a fast rider, and that his molar or canine tooth will be like Mount Uhud, while the thickness of his skin will be equivalent to a three-day journey, as reported in Sahih Muslim and al-Bukhari.
As for the people of Paradise, they will enter it in the most beautiful form and appearance. They will be in the form of their father Adam, peace be upon him, sixty cubits tall, as the Prophet ﷺ said:
“In the form of a single man, in the image of their father Adam, sixty cubits tall.” Narrated by al-Bukhari and Muslim.
They will also enter Paradise hairless, beardless, with kohl around their eyes, at the age of thirty or thirty-three years. They will neither grow old nor suffer from weakness or illness.
Thus, resurrection will take place by restoring the human being with his body. After the reckoning, his final destination will be either Paradise or Hellfire, each having the characteristics described by the Prophet ﷺ.
And Allah knows best.