It is mandatory (fardh) to wash from forehead to bottom of chin in wudu. But how will you define bottom of chin ? How much area does it cover below the chin ? Is it up to bone of chin only or is it cover more than that ?

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, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Linguistically, the face is the part that which the human being faces others with; its limit vertically is from the forelock at the top of the forehead of a normal haired man to the bottom of the chin, which is the end of the two sides of the jawbone. The jawbone is the bone where the lower teeth grow. The front end of the bone where the lower teeth grow is the lower end of the limit of the face.

Al-Khateeb Ash-Shirbeeni from the Shaafi’e School of jurisprudence, said: “The limit of the face vertically is between the roots of the front hair of the head and the lower side of the front end of the two sides of one’s jawbone where the lower teeth grow.”

Besides, in his "Minah Al-Jaleel", ‘Ulaysh from the Maaliki School of jurisprudence, said: “The end of the chin is the front end of the two sides of the jawbone for whoever does not have a beard, like a woman or a beardless man, or the end of the front side of the beard for whoever has a beard. So, it includes the chin and the beard, and the front side of the beard is its part which appears when facing someone. However, its (the beard's) inner part which is facing the chest does not need to be washed, and this is an innovation and an exaggeration in religion, and it is a surplus to the part whose washing is obligatory, so it is disliked to wash it.” [Abridged]

Based on the above, the limit of the face has become clear to you, which is the part that extends to the end of the chin which is the (front end of the) bone where the lower teeth grow.

Allaah Knows best.