Hello Steven,
That is a question of personal preference.
A 6 inch buckle receiver should do just fine.
However, as Tyler just mentioned, a flexible cable is much more recommended.
Or ... if yours is stiff, get it bended and a bit angulated towards the seat cushion.
Mine are 22 cm (= 8.6 inches) long, and I personally find them a bit too long. But they're flexible and that is definitely a plus.
Good luck
Achim