HAZRAT MAHDI (AS) WILL HAVE A VERY POWERFUL BODY AND VOICE; AT THE SAME TIME HIS WORDS AND SPEECH WILL HAVE A GREAT IMPACT ACROSS THE WORLD

Hazrat Riza (as) replied to Rayyan Ibn Salti when he asked whether he were the commander of the faithful: Yes, I am also the commander of the faithful. But I am not the commander of the faithful who will fill the world with justice. The promised Qaim [Hazrat Mahdi (as)] will arise advanced in age, but will have a youthful appearance. HE WILL BE SO POWERFUL THAT IF HE STRETCHES HIS HAND OUT TO THE LARGEST TREE IN THE WORLD HE WILL UPROOT IT AND CAST IT AWAY, AND IF HE SHOUTS AMONG THE MOUNTAINS, HARD ROCKS WILL TURN INTO POWDER.  He will have Musa's (as) staff and Sulayman's (as) ring... Allah will keep him hidden from sight until He wills. He will then appear and will fill the world with justice where it was formerly filled with oppression. (Bihar Al-Anwar, Vol. 52, p. 32; Ithbat Al-Hudat, Vol. 6, p. 19)

The account states that Hazrat Mahdi (as) is physically powerful enough to uproot a tree or to turn rocks into powder with his voice. It is also clear that it is not referring to Hazrat Mahdi (as) having superhuman and metaphysical powers enabling him to really uproot a tree or to demolish mountains and rocks just by speaking.

Language of this kind is commonly used in colloquial speech. People often use terms like "strong as a lion" to refer to someone physically strong.  But these are always used in a figurative sense. Otherwise, it is not really possible for the person to be "as strong as a lion". These are merely metaphors used to emphasize that someone of physically powerful and healthy.

There are a great many trustworthy hadith and accounts narrated from our Prophet (saas) and the people of the Book (Ahl al-Bayt) describing characteristics of Hazrat Mahdi (as) in metaphorical or figurative terms. This hadith is one such. In the Qur'an, Allah says that He strengthens His messengers in body and knowledge.

...He said, "Allah has chosen him over you and favored him greatly in knowledge and physical strength. Allah gives kingship to anyone He wills. Allah is All-Encompassing, All-Knowing." (Surat al-Baqara, 247)

This manifestation of Allah will be seen in Hazrat Mahdi (as), and Hazrat Mahdi (as) will have a stronger body and voice than most people.

However, the metaphor in the above account of him being able to move mountains and rocks with his voice also indicates that Hazrat Mahdi's (as) words and speech will be full of wisdom and shows that they will have a powerful impact on everyone in the world.