http://www.dailywritingtips.com/body-parts-as-tools-of-measurement/

English possesses several measurement words that derive from body parts.
cubit – from Latin cubitum, “the elbow.” A cubit was a measurement based on the forearm from elbow to fingertip. The exact length varied according to whose arm was being used and could be from 18 to 22 inches.
digit – In Latin, digitus could mean either “finger” or “toe.” The same is true of digit in modern English. People use the digits of their hands to count to ten. And just think, we imagine we’re so modern and up-to-date because we live in a Digital Age.
fathom – Old English faethm was “the length of the outstretched arm,” about six feet. Water depth is measured in fathoms. Miners use the term to describe an area equal to six square feet.
hairbreadth – According to the Online Etymology Dictionary, hairbreadth is said to have once been a formal unit of measure equal to one-forty-eighth of an inch.
hand – Originally, a “hand” was a measurement of three inches, but now it is four inches. This measurement is stil used to reckon the height of horses.
handful – This is an indeterminate quantity of some dry measure, such as grain, that can be held in the cupped hand. A Greek word for “handful” is the origin of the Greek coin called a drachma.
span – In Old English a span was “the distance between the thumb and little finger of an extended hand,” roughly nine inches. Again, it all depended on whose hand.
thumb was probably the basis of the measurement now called an inch. We don’t measure things in “thumbs,” anymore, but we do talk about thumbnail sketches and thumbnails in the sense of “small images.”
The expression rule of thumb probably originated with carpenters who used their thumbs to take rough measurements. The notion that the expression originated with a law permitting a husband to beat his wife with a stick “no larger in circumference than his thumb,” has no historical basis.