How to Wash the Face in Wudu
Question

I read that the boundaries of the face for Wudu are: from the hairline to the chin in length, and from ear to ear in width. Is there evidence for this, or is this a matter of Ijtihad from the scholars?
Praise be to Allah.These are the boundaries of the face upon which the scholars have agreed, and they are its limits from the perspective of the language in which the Quran was revealed, thus the description and the boundary are based on two legal evidences:

From the standpoint of the scholars’ agreement and consensus, their consensus is a proof.
And from the standpoint of the language in which the Quran was revealed, and we are addressed by it, and there is no contradiction to it from the perspective of the Shari`ah.

Allah the Almighty says (interpretation of the meaning): {O you who have believed, when you rise to [perform] Salah, wash your faces and your forearms to the elbows and wipe over your heads and wash your feet to the ankles} [Al-Ma’idah 5:6]
The linguists say: “The face is the front of everything.” (End quote) (Al-Muhit fil-Lughah, 1/314; Kitab Al-`Ayn, 4/66)
Al-Qurtubi (may Allah have mercy on him) said:
“The face in language is derived from facing, and it is a member that includes other members and has length and width; its boundary in length is from the beginning of the forehead to the end of the jawbones, and from ear to ear in width.” (End quote from Al-Jami` li Ahkam Al-Quran, 6/83)
He also said:
“And the Arabs do not call a face except what is met with facing.” (End quote from Al-Jami` li Ahkam Al-Quran, 6/84)
Ibn Kathir (may Allah have mercy on him) said:
“And the boundary of the face according to the Jurists: is from where the hair of the head grows – and there is no consideration for baldness or hair density – to the end of the jawbones and the chin in length, and from ear to ear in width.” (End quote Tafsir Ibn Kathir,  3/47)
Ash-Shirazi (may Allah have mercy on him) said:
“Then he washes his face, and that is mandatory due to the saying of the Almighty}(translation of the meaning: {wash your faces)
And the face is what is between the hairline of the head to the chin and the end of the jawbones in length, and from ear to ear in width.” (End quote)
An-Nawawi (may Allah have mercy on him) said:
“What the author mentioned regarding the boundary of the face is the correct view which the companions adhere to and Ash-Shafi`i (may Allah have mercy on him) explicitly stated in Al-Umm.” (End quote from Al-Majmu`, 1/405)
An-Nawawi (may Allah have mercy on him) also said in Al-Majmu` (1/399):
“And the face, according to the Arabs, is what is met with facing.” (End quote)
Al-Kasani (may Allah have mercy on him) said in Bada’i` As-Sanai` (3/1):
“And the apparent narration did not mention the boundary of the face, and it mentioned in another narration of the foundations that it is from the hairline to the bottom of the chin, and to the lobes of the ears, and this is a sound definition; because it defines the thing by what the word indicates linguistically; because the face is a name for what a person faces, or what is customarily faced to, and the facing occurs with this defined area.” (End quote)
Refer to: Daqa’iq Uli An-Nuha, 1/56; Kashshaf Al-Qina`, 1/95; Al-Mughni, 1/83; Tabayyun Al-Haqa’iq, 1/2; Fat-h Al-Qadir, 1/15; Matlab Uli An-Nuha, 1/113; Radd Al-Muhtar, 1/96; Al-Mawsu`ah Al-Fiqhiyyah, 4/126; Tafsir Ibn Kathir, 3/48; Al-Kulliyat; 1628; Al-Lubab, 7/219; Tafsir Al-Baghawi, 3/21; Nazm Ad-Durar, 403/2.
Thus, the statements of the exegesis, Jurists, and linguists have converged on the fact that the face is what is met with facing, and that this is its boundary. And that is sufficient as a legal proof.
For more details, please see the following answers: 11497, 71169, 85031, 226422, 426969, 88179.
And Allah the Almighty knows best.