WIre length in Conducted emission test
It is preferable to use a standard unbundled 1m length of cable for
  these tests, whatever the length that will be supplied with the final
  product.


In other words: -



That is the advice given on this website (Guide to Testing Conducted Emissions (Based on the Methods in EN 55022 and EN 55011) – Part 2) for performing conducted emission tests.

Clearly, if you had a conducted emission problem, this could be "mitigated" by using a much longer cable therefore the 1m requirement has the intention of putting everyone on a level playing field. 

Different specifications may have different cable-length requirements.