Praise be to Allah.

Al-Bukhaari (917) and Muslim (544) narrated from Abu Haazim that a group of people came to Sahl ibn Sa‘d, and they had differed concerning the minbar and what kind of wood it was made of. He said: By Allaah, I know what kind of wood it is made of, and who made it, and I saw the Messenger of Allaah (blessings and peace of Allah be upon him) the first day he sat on it. I said to him: O Abu ‘Abbaas, tell us. He said: The Messenger of Allaah (blessings and peace of Allah be upon him) sent word to a woman of the Ansaar, saying: “Let your carpenter slave made me something of wood from which I may speak to the people.” So he made these three steps, then the Messenger of Allaah (blessings and peace of Allah be upon him) ordered that it be placed in this spot. It was made of tamarisk wood from al-Ghaabah (a wooded area near Madeenah). 

Tamarisk is a tree that has no thorns. Al-Ghaabah is a well-known place near ‘Awaali al-Madeenah. What is meant is that it was made of this wood. 

Ahmad (2415) narrated that Ibn ‘Abbaas said: The minbar of the Messenger of Allah (blessings and peace of Allah be upon him) was short, only three steps. Its isnaad is hasan. Al-Haythami said: It was narrated by Ahmad and its men are the men of as-Saheeh. Majma‘ az-Zawaa’id, 2/386 

Muslim (1017) narrated from the hadeeth of Jareer: He (i.e., the Prophet (blessings and peace of Allah be upon him) prayed Zuhr, then he ascended a small minbar. 

Al-Daarimi (41) narrated from Anas ibn Maalik that the Prophet (blessings and peace of Allah be upon him) used to stand on Fridays, leaning his back on the trunk of a palm tree that had been set up in the mosque, and he would address the people. Then a Byzantine came and said: Shall I not make for you something on which you may sit, and it will be as if you were standing. So he made for him a minbar that had two steps and he would sit on the third. When the Prophet of Allah (blessings and peace of Allah be upon him) sat on that minbar, the palm trunk made a groaning sound like the lowing of a bull, until the mosque shook, out of sorrow for (the departure of) the Messenger of Allah (blessings and peace of Allah be upon him). So the Messenger of Allah (blessings and peace of Allah be upon him) came down from the minbar and embraced it while it was groaning, and when the Messenger of Allah (blessings and peace of Allah be upon him) embraced it, it fell silent. Then he said: “By the One in Whose hand is the soul of Muhammad, if I had not embraced it, it would have carried on like that until the Day of Resurrection, out of sorrow for (the departure of) the Messenger of Allah (blessings and peace of Allah be upon him).” 

Narrated by Ahmad (21252) and ad-Daarimi (36) from the hadeeth of Ubayy ibn Ka‘b. 

Al-Albaani said in as-Saheehah (3/175): Its isnaad is jayyid and it meets the conditions of Muslim. 

Abu Dawood (1082) narrated that Salamah ibn al-Akwa‘ said: Between the minbar of the Messenger of Allah (blessings and peace of Allah be upon him) and the wall was a space that was sufficient for a sheep to pass through.

Classed as saheeh by al-Albaani in Saheeh Abi Dawood. 

Al-Haafiz Ibn Rajab (may Allah have mercy on him) said: 

The correct view is that the minbar had three steps, and it remained like that during the time of the Rightly Guided Caliphs. A number of scholars regarded increasing the height of the minbar as an innovation, including Ibn Battah among our companions, and others. Some of the Shaafa‘is regarded the very large minbar as makrooh, if it is too big for the mosque.

End quote. Fath al-Baari by Ibn Rajab, 8/242 

Ibn Hajar (may Allah have mercy on him) said: 

The minbar remained as it was, with three steps, until it was increased to six steps by Marwaan during the caliphate of Mu‘aawiyah. End quote. 

Fath al-Baari by Ibn Hajar, 2/399 

An-Nawawi (may Allah have mercy on him) said: 

The scholars said that the minbar of the Prophet (blessings and peace of Allah be upon him) was three steps, as was stated clearly by Muslim in his report. 

From the saheeh hadeeths quoted above, it is known that the minbar of the Prophet (blessings and peace of Allah be upon him) was small, short and humble, made of wood, with three steps. The Prophet (blessings and peace of Allah be upon him) used to deliver the khutbah from the second step and sit down on the third. And the distance between his minbar and the wall was a space that was sufficient for a sheep to pass through. So it did not break up a row, and did not bother anyone. Rather it was a humble wooden structure composed of three steps, with no adornment or engravings, and no exorbitant expenditure. This is how the minbars should be in the mosques of the Muslims. 

You have done well in your desire to resize this minbar and make it three steps. We ask Allah to guide you and help you to act in accordance with the Sunnah.