Minimum frame size in ethernet
This might not be the correct answer to your question(as I am not sure about jamming signal), but it might resolve the query.

As Ethernet uses CSMA/CD for error detection, the time taken to transmit the data must be greater than or equal to twice the time taken to propagate the data. Only then the station will come to know if there had been a collision.

Tt &gt;= 2*Tp