How long was the German front at its furthest extent when in Russia in
December 1941? In June 1941, the length of the front of the German
line was 1,800 to 2,000 miles. What was it by December 1941? What was
it at its maximum?

http://history.acusd.edu/cdr2/WW2Pics/82422.jpg
indexed by:
http://history.sandiego.edu/gen/WW2Index/picindexmapsi.html

This is a map as at end March 1942, but it also shows the German lines
in the first week of December 1941
- a bit of damp cotton should come up with the length.

However it is not so much the length of lines that matters, more where
they are and which way they are moving.

According to a speech by Russian Ambassador at Australia, L. Moiseev,
the maximum length of the Eastern Front was 6200 kilometers:
http://www.australia.mid.ru/canberra_e_01.html