Purity: The Width of Foot Wiping
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 252
The wiping of the feet can be of any width; however, it is better that the width be the width of three fingers joined together; and it is even better for the whole of the upper foot to be wiped with the whole palm.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)