Trimming the beard less than a fist length

Growing a beard is fard and it is sinful to shave it. The Prophet (Sallallahu Alayhi Wasallam) commanded us to grow them. If the hadith of the beard was to be looked at technically, the wording in the hadeeth is to be understood to let the beard grow. As in just leave it completely. But we have narrations of sahaba trimming their beards a fist length. So scholars have concluded this is the exception to the case and have set a minimal length of the beard to be a fistful. Who ever trims it so its smaller than a fist length is a sinner. Is this correct? I do see many people of knowledge who trim it so its less than a fist length. Please elaborate.