Purity: The Area of Head Wiping
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 249
The area that must be wiped is the front quarter of the head, i.e. the quarter immediately above the forehead. It is sufficient to wipe any part of this area and to any extent, although the recommended precaution is that the length of the wiping should be at least the length of one finger, and the width of the wiping should be at least the width of three fingers joined together.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)