Praise be to Allah.
How tall was Adam (peace be upon him)?
This hadith was narrated by Abu Hurayrah (may Allah be pleased with him) from the Prophet (peace and blessings of Allah be upon him) who said:
“Allah created Adam and he was sixty cubits tall. Then He said, ‘Go and greet those angels and listen to how they greet you, for that will be your greeting and the greeting of your progeny.’ He said, ‘Al-salamu ‘alaykum (Peace be upon you).’ They said, ‘Al-salamu ‘alaykum wa rahmat-Allah (Peace be upon you and the mercy of Allah).’ So they added the words ‘wa rahmat Allah.’ And everyone who enters Paradise will be in the form of Adam. People kept on growing smaller until now.” (Narrated by al-Bukhari, 3336; Muslim, 7092)
According to a version narrated by Muslim: “Everyone who enters Paradise will be in the form of Adam who was sixty cubits tall. People kept growing smaller until now.”
With regard to the words of the Prophet (peace and blessings of Allah be upon him), “People kept growing smaller until now,” al-Hafiz ibn Hajar said in Fath al-Bari (6/367): “This means that in every generation people grew shorter than the previous generation, and continued to grow shorter until the time of this ummah, then they stayed like that.”
The Muslim is obliged to believe in every idea for which there is evidence in the Quran or sahih Sunnah from the Messenger of Allah (peace and blessings of Allah be upon him). Imam al-Shafi'i (may Allah have mercy on him) said: “I believe in Allah and in that which came from Allah in the sense meant by Allah. I believe in the Messenger of Allah and in what came from the Messenger of Allah in the sense meant by the Messenger of Allah.” See al-Irshad Sharh Lam’at al-I’tiqad, p. 89.
So the believer is required to believe with firm faith in everything that we are told by Allah and by the Prophet (peace and blessings of Allah be upon him), if it is proven to be soundly reported from him (peace and blessings of Allah be upon him). He must believe in it with firm faith that leaves no room for the slightest doubt. He must accept it in general and specific terms, whether he understands it or not and whether he finds it strange or not, because not understanding something that is proven in a sound report does not mean that it did not happen. All that means is that he cannot comprehend this particular issue. Allah has commanded us to believe in everything that He tells us and everything that His Prophet (peace and blessings of Allah be upon him) tells us. Allah says (interpretation of the meaning):
“Only those are the believers who have believed in Allah and His Messenger, and afterward doubt not but strive with their wealth and their lives for the Cause of Allah. Those! They are the truthful” [al-Hujurat 49:15]
Is belief in the unseen an article of faith?
Part of faith is belief in the unseen (al-ghayb). The hadith we are discussing here comes under this heading. Allah praises those who believe in the unseen, as He says (interpretation of the meaning):
“Alif-Lam-Mim. [These letters are one of the miracles of the Quran and none but Allah (Alone) knows their meanings.]
This is the Book (the Quran), whereof there is no doubt, a guidance to those who are Al-Muttaqun [the pious believers of Islamic Monotheism who fear Allah much (abstain from all kinds of sins and evil deeds which He has forbidden) and love Allah much (perform all kinds of good deeds which He has ordained)].
Who believe in the Ghayb[the unseen].” [al-Baqarah 2:1-3]
Belief in the Omnipotence of Allah
You should note that Allah is Able to do all things. Just as He is able to create man in the form that he appears in now, He is also able to create man in a larger or smaller form.
If this is difficult for you to understand, then think of the dwarves that we see, who are child-sized men. If this can happen, then why could the opposite not happen, namely a man being sixty cubits tall? In the history of mankind there have been giants as the archaeologists tell us.
The basic principle here is to accept the absolute power of Allah and to accept what He tells us and what His Messenger (peace and blessings of Allah be upon him) tells us, and to say what those who are well versed in knowledge say:
“We believe in it; the whole of it (clear and unclear Verses) are from our Lord.” [Al 'Imran 3:7 – interpretation of the meaning].
We ask Allah to show us the truth as true and help us to follow it, and to show us falsehood as false and help us to avoid it.
For more about some issues related to Prophet Adam (peace be upon him), please see these answers: 8492 , 20652 , 4622 , and 170025 .
And Allah knows best.