My question is about the size of beard. Is it haram to trim my beard? Were there people among the companions who trimmed their beard than a fist?

Trimming the beard is a Sunnah. The Prophet (peace and blessings be upon him) used to trim his beard and cut some of its sides and the top portions in order to match the contour of his face in order to go well with his overall appearance. He also used to wash his beard with water and comb it. It is also known that the Companions of the Prophet followed him in his actions and decisions.