How long the beard should be
The beard has a role in Islam. It is an obligatory (Fard) Sunnah of the Prophet (ﷺ). When growing the beard, it is important to know how long the beard should be.
There are no authentic reports that indicate exactly how long the beard of the Prophet (ﷺ) was. It is confirmed that it was a long and full beard.
Abu Ma'mar reported: I asked Khabbab bin Al-Arat if the Prophet (ﷺ) recited the Quran in the Dhuhr and 'Asr prayers. He answered in the affirmative. I said, "How did you find out?" He replied, "By the movement of his beard." [Sahih al-Bukhari 761]
Jarir ibn Samura reported: "The Prophet (ﷺ) had a full beard." [Sahih Muslim 2344b]
By looking at the Companions, we find out how long a beard must be. The Companions had long beards. What was the exact length that the Companions maintained? Various narrations indicate that their beards maintained a fist length.
The Companions shortened their beard if it grew too long. It is therefore not mandatory that one grows one's beard to the knee.
The correct length of a beard is therefore at least a fist length. Make a fist of your hand and place it under your chin. That is the minimum length of the Muslim beard. If the beard gets longer than a fist length, then that is okay too. When the beard gets too long, it is trimmed to a certain length.
It is therefore only allowed to shorten the length of the beard if it has exceeded a fist length. For example, cutting anything shorter than that is not allowed. That is according to the saying of the Prophet (ﷺ).
The Prophet (ﷺ) said: "Cut the mustache short and let the beard grow." [Sahih al-Bukhari 5893]