Sentence:  If , in a certain part of the range , it starts life 1 foot longer than do any of the other ( relatively large ) giants , and reaches maturity at , let us guess , 18 inches longer than the others , a quadrupling of the maturity <WSD>length</WSD> would result in a maximum of ( nearly ) 40 feet . 
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