Sentence:  The <WSD>length</WSD> of the arrow indicates amount of slowing or acceleration at Completion over that at Onset , and the difference in months can be read roughly by referring the arrow to the age scale along the base of each figure , or more precisely by referring to the original data in the appropriate tables . 
Meaning of the word marked with <WSD> tags:  the linear extent in space from one end to the other; the longest dimension of something that is fixed in place