Hazrat Mahdi (as) will be of medium stature

Hazrat Mahdi (as) will be of average height. (Ibn Hajar al-Haythami, Al-Qawl al-Mukhtasar fi `Alamat al-Mahdi al-Muntadhar, p. 41)

According to accounts by Abu Abdullah Nuaim Ibn Hammad from Abu Ja’far bin Muhammad bin Ali al-Baqir, Hazrat Ali was asked about the attributes of Hazrat Mahdi (as) and he replied: “He [Hazrat Mahdi] is of medium  height and a young man with a beautiful face…" (Al-Uqayli, An-Najmu's-saqib fi Bayan Anna'l Mahdi min Awladi Ali b. Abu Talib Ale't-Tamam wa al-Qamal) 

Our Prophet (saas) is also describedas being of medium height in accounts: 

Anas bin Malik stated that: "The Messenger of Allah (saas) was of middling height."(At-Tirmidhi, Imam Abu `Isa Muhammed ibn `Isa Tirmidhi, Shamail Sharif, p. 15)