Narrated Abu Huraira:
The Prophet (ﷺ) said,
“Allah created Adam, making him 60 cubits tall. When He created him, He said to him, “Go and greet that group of angels, and listen to their reply, for it will be your greeting (salutation) and the greeting (salutations of your offspring.” So, Adam said (to the angels), As-Salamu Alaikum (i.e. Peace be upon you). The angels said, “As-salamu Alaika wa Rahmatu-l-lahi” (i.e. Peace and Allah’s Mercy be upon you). Thus the angels added to Adam’s salutation the expression, ‘Wa Rahmatu-l-lahi,’ Any person who will enter Paradise will resemble Adam (in appearance and figure). People have been decreasing in stature since Adam’s creation.
According to Muhammad Asad,
“This story of Adam is obviously one of the allegories referred to in 3:7”
The tallness mentioned in this narration is metaphorical and is used to describe the innocent and God guided state of the early people. It is this state that has degenerated during the years, in that humanity has slowly overtime become less and less conscious towards the divine revelaion.
The following Quranic verse explains the same idea:
“Do ye wonder that there hath come to you a message from your Lord through a man of your own people, to warn you? call in remembrance that He made you inheritors after the people of Noah, and gave you a stature tall among the nations. Call in remembrance the benefits (ye have received) from Allah: that so ye may prosper.” (Quran 7:69)
What this verse is describing of the people of AD is not their physical height but their spiritual/economic strength as a nation.