How to measure the actual length of a long twisted pair?
From a purely geometric perspective you could calculate the length using the helical length equation. 



Where H is the length of the twisted wire and

 

Where R equals the radius of the turns in the wire. Basically from the center of the twisted pair assembly to the center of one of the wires.



So if the wire makes a complete rotation around the center in 10mm, and distance between the center of the twisted pair and the center of a wire is 1mm, then were you to untwist the wire and straighten it the length would be