In Shiva Sutra, III.4 it is mentioned about Pranavayu that
this is a way of meditation to concentrate on Prana i.e. on the expiration and inspiration of breath from the centre of the body upto a distance of twelve fingers in the external space. The mind has to be concentrated on the starting point of the breath from the centre and the ending point at a distance of twelve fingers from the center which is known as the end of dvaadashaanta the distance of twelve fingers. The external distance is known as baahya dvaadashaanta and internal (the starting point) is known as antar dvaadashaanta . One has to concentrate on the points Where the expiring and inspiring breaths stop for a split seconds.
My question is what is twelve fingers in this context ?