Why would you take 6’ of chain? Wouldn’t 2-3’ be better?
Not being a student of martial arts, I've never considered a chain to be a weapon before. However, it seems to me that a swinging chain would be a decent weapon against a knife or razor. Getting your arm or head whacked by a swinging chain would definitely not make your day.
It seems to me that it would need to be long enough that the attacker could not reach your arm with his razor, but not so long that it would be hard to reaccelerate it back to rotating. Six feet seems to me to be about right.