All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
The questioner probably means the narration of Jaabir who said, "We used to let the beard grow except in Hajj and 'Umrah." [Abu Daawood]
There is no mention of a fist and what increased the length of the fist in the above narration.
The scholars have different opinions concerning the authenticity of this narration. Some classified it as sound while others considered it as weak. However, the narration is not very weak since the chain of the narrators is free from liars or fabricators.
So, the scholars have different opinions about trimming the beard that exceeds the length of the fist, some of them permitted it. It is proved that Ibn 'Umar and Abu Hurayrah used to trim what exceeds the length of the fist. A group of scholars prohibited it for the narrations that generally prohibit cutting the beard.
Imaam, An-Nawawi said, 'It is better to let the beard grow as it is, and should not make it short at all.'
Allaah Knows best.