The Mahdi will be of average height.
(al-Haythami, Al-Qawl al-Mukhtasar, p. 41)
Anas stated that: "The Prophet (saas) was of middling height." As we know, the word "rab'a" in the hadith means normal, medium stature. But there is a limit to normal height in the eyes of tall people, because normal height refers seven handspans.
(Hadith at-Tirmidhi, Shamail Sharif, p. 15)