The musjid was 100 cubits long (approximately 45.7m) and approximately 100 cubits wide. The foundations were about 3 cubits (approximately 1.37m) deep. The height was slightly higher than the height of an average man. The qiblah wall was facing Bayt-ul-Muqaddas (in Jerusalem).

Three doors were built in the musjid. One door was situated on the side where the qiblah is today. The second door was positioned on the western wall, and is today referred to as Baab-ur-Rahmah. The third door was the door frequently used by Rasulullah (sallallahu ‘alaihi wasallam), and is today referred to as Baab-u-Jibreel.

After about sixteen or seventeen months, when the qiblah direction of Bayt-ul-Muqaddas was abrogated (cancelled), and the law was revealed commanding the Muslims to perform salaah in the direction of the Holy Ka’bah, then the door at the back of the musjid was sealed off (as this now became the qiblah wall) and another door was made directly opposite it (as this was now the rear of the musjid).

Musjid-un-Nabawi underwent construction twice (during the blessed lifetime of Rasulullah [sallallahu ‘alaihi wasallam]).

The first construction was at the time when Rasulullah (sallallahu ‘alaihi wasallam) performed hijrah and was residing in the home of Hazrat Abu Ayyoob Ansaari (radhiyallahu ‘anhu).

The second construction was in the year 7AH, after the Conquest of Khaibar. At this time, Rasulullah (sallallahu ‘alaihi wasallam) rebuilt the musjid on account of the structure becoming old.

In the first construction, the length and breadth of the musjid was less than 100 cubits, and in the second construction, the length and breadth of the musjid was over 100 cubits.

(Extracted from Seeratul Mustafa 1/427-428)