I know thay having a long beard is a sunnah, but my mother and office says that having a beard is sunnah but they want me to have a short beard.So would it be a sinful for me have a short beard?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

It is Sunnah to leave your beard without having cut at all. But, according to the majority of Muslim scholars, there is no harm in having cut what exceeds one's fist, especially if it is done to give your mother pleasure. Also, according to the majority, it is Haram to shave it.
Allah knows best.