What is torsional limit?
Torsional limit is the amount of rotational torque that can be applied to a "locked" instrument to the point of breakage (separation). 
Obviously, an instrument should have sufficient strength to be rotated and worked vigorously without separating in the canal.
Smaller instruments (less than size 20) can withstand more rotations without breaking than larger (greater than size 40) instruments.

Ref :Principles and Practice of Endodontics, Richard E Walton, 3rd edition, pg no:156