Praise be to Allah.
Allaah created Adam (peace be upon him) sixty cubits tall, then mankind gradually grew shorter until they stopped and remained as they are now. The evidence for that in the Sunnah is the hadeeth of the Prophet (peace and blessings of Allaah be upon him): “Allaah created Adam sixty cubits tall, then mankind kept getting shorter until now.” (Narrated by al-Bukhaari, 3326; Muslim, 2841). Ibn Abi Haatim narrated with a hasan isnaad from Ubayy ibn Ka’b that the Prophet (peace and blessings of Allaah be upon him) said: “Allaah created Adam a tall man with a lot of hair on his head, as if he were a tall palm tree.”
Al-Haafiz ibn Hajar (may Allaah have mercy on him) said in Fath al-Baari:
“ ‘Mankind kept getting shorter until now’ means that each generation grew shorter than the generation before, and that decrease in height ended with this ummah, and that is how they stayed.”
And Allaah knows best. May Allaah send blessings and peace upon our Prophet Muhammad.