Nick.
If you do mean the hooklength, then going on what has been said on past threads, it is best to experiment on different waters, lake bed conditions, and situations and hooklength materials. I personally have always used hooklengths around ten inches, for all situations and conditions and caught my share of carp on gravelly bottoms or in deep silt, but I am not as into carp fishing as some of the lads on here are.
Last week I was with one carper who only uses hooklengths of six inches, and he catches more than anybody else. But he also knows the particular water that he fishes on like the back of his hand, which may or may not be more important than the hooklength.
Incidentally, he uses a stiff hooklength with a braid hair tied seperately.