The Western Front at it's most consistent, from December 1914 to March 1918, was about 540 miles in length. The Eastern Front was just over twice that length. Trench lines typically ran continuously for miles on end, and it was only in areas such as the Vosges sector on the southern most part of the Western Front, or in areas of the Eastern Front such as the Pripet Marshes where continuous lines were unfeasible, that there were breaks in the line. The short answer is that trench length varied greatly depending on the theatre and the circumstances. A simple, temporary battlefield trench might be long enough to contain a section of men, so 10+. But again, the length more often than not exceeded this, and so did the complexity of the trenches themselves.

A good place to start would be Nicholas Murray's book, Rocky Road to the Great War, which is also the basis of [this lecture] (https://www.youtube.com/watch?v=2cbq7iu8FrI).