Purity: Specifications of the Shroud (Kafan) Pieces
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 559:
Based on obligatory precaution, the loincloth must cover the area from the navel to the knees; and it is better that it cover the area from the chest to over the feet. Furthermore, based on obligatory precaution, the shirt must cover the area from the top of the shoulders to the middle of the calf; and it is better that it reach over the feet. As for the full cover, this must be long enough to cover the entire body; and the obligatory precaution is that its length must be long enough for both ends to be tied, and its width must be wide enough for one side to overlap the other.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)