Your handspan is a measurement of the size of your hand. You take it by placing your little finger on the edge of a table, and stretching your thumb as far onto the table as you can, without removing your finger from the edge. You then note where the end of the thumb is, and measure to it from the edge of the table.
That is, your handspan is the distance from the tip of your little finger to the tip of your thumb, when your hand is stretched as wide as you can get it.
My handspan is 25cm, just short of 10 inches. I seem to have unusually large hands.
I can't, off hand (all puns intended), think of many applications for hand span measurements, but they are popular with primary school maths books (in the UK) to get children to measure things.
Not just a nodeshell, a nodeshell from Pseudo_Intellectual!