Jasminsandhu ji

The Sikh Rehat Maryada does not say how long a kirpan should be or how it should be worn, only that a practicing Sikh should wear a kirpan as part of the 5 kakkars. So your elders were wise not to make up an explanation based on hear-say.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/sikh-sikhi-sikhism/31177-a-question-my-elders-cannot-answer.html
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=31177

I did a little searching and could not find the answer for you. My own inclination is like your elders : not to invent.

Maybe another forum member will be able to answer from the perspective of history. Good luck.