I have caught fish on some 18 inch worms that I poured, but it took awhile caue every time I would swing I would get half a worm. I have done best using 10 in and 12 in. , but I havent noticed any larger fish on average. I use them at night to make more vibration. In the daytime I get bit more and by the same sized fish with a 7 in,
The one exeception is senko's I get better fish on the larger sizes. I even made a 12 version and did pretty good with it.
Matt