The tag end will be something your'll have to figure out on that day your out, but i'd start at 18" and work your way down to 6". I like to use robo worms most of the time ,but found that a shad body bait is good to. I personal think yamamoto shads are very good, So are there cut tail worms.
Here is a link to all you'll ever need to know about drop shotting. Rich is one the best at it.
http://www.richz.com/fishing/articles/dropshot.html