According to Sayed Sistani: Based on obligatory precaution it is not permissible for a man to shave his beard. Similarly, it is not permissible for him to just leave the hair on his chin and shave the sides. This too is based on obligatory precaution. (See the question-answer section below.)A Muslim is allowed to shave his beard, if he is compelled to do so or if he is forced to shave it for medical reasons, etc. It also allowed if he fears harm to his life by not shaving or if growing the beard would put him in difficulty (for example, if it becomes a cause of ridicule and humiliation that is not normally tolerable by a Muslim).
We see that it is impermissible to have a clean shaven face and also a goatee. So this shows us that the beard should be complete. As far as length it should be enough to be considered that he has a beard even if it is very low.