According to Top End Sports, the hand is placed palm down on a flat surface. The fingers are outstretched as far as possible. Measure the linear distance between the outside of the thumb to the outside of the little finger.
Height, weight, arm length, and hand span are important for defensive linemen, but in different ways for each position along the defensive line. Big hands and long arms are a plus wherever a defensive lineman plays, allowing them to get their hands on an offensive lineman first and either control him or shed the block. Read more at Big Blue View.