Hi Mouse,
It depends what you are going to use the worm for. The best is to cut the worm up into pieces and use only about 2/3cm at a time. I found this size is the most effective for blacktail etc. Anything bigger they just take that tail off and smaller doesn't work that well. This is all obviously relative to the hook size. I normally fish them on a 1/8th ounce. If using a bigger jig head then get the 2", but the 6" is more versatile, you can match it to the hook.
Retrieve is normally slow along the bottom. I have experimented with other retireves and found also a faster middish water retrieve also quite successful. If there are lots of rocks around you don't want to go to slow as you will get caught up on the bottom. I have even caught a small kingie on the mid water retrieve. That was on the red (bloody) worm. I have also tried to do a weedless version, to stop it from getting on the rocks. Still need more time trying it out to see if it works. I know you can also get weedless hooks now. Maybe try that on the worm.
Cheers,
Bluefin