The hadiths relate that Hazrat Mahdi’s (as) brow will be open and broad, and that his head will be proportionately large.
Hazrat Mahdi (as) is one of my people ... His brow is open. (Al-Muttaqi al-Hindi, Al-Burhan fi Alamat al-Mahdi Akhir az-Zaman, p. 21)
Allah will send a son of my line [Hazrat Mahdi (as)], with a clear brow, who will fill Earth with justice and distribute goods and property in abundance. (Ibn Hajar al-Haythami, Al-Qawl al-Mukhtasar fi `Alamat al-Mahdi al-Muntadhar, p. 23)
Certain it is that Allah will send a person [Hazrat Mahdi (as)] from my line with a clear brow. (Mari’ bin Yusuf Karami Hanbali, Fawaid Fawaid al-Fikr fil Mahdi al-Montazar, p. 11)
He [Hazrat Mahdi (as)] will have a clear brow… (Jalaluddin as-Suyuti, Kıyamet alametleri, Olum ve Dirilis [Portents of the Judgment Day, Death and Resurrection], p. 174)
Hazrat Mahdi (as) has a clear brow. (This hadith is narrated in Sunan Abu Dawud and Mustadrak al-Hakim.)