My long arm manufacturer recommends size 18 needles for everything.....I generally stitch with 40wt poly and occasionally with king tut cotton. The 18s work fine with them.
When I work with the thinner threads (so fine when I can get it to work) I use size 16. Havent tried using silk thread yet, so havent had a need for the smaller needles.
I will say this, I have heard all kinds of advice on how to work with batiks. From needle sizes, stitch lengths, thread types, how to prep for piecing and quilting. I have no idea where these "rules" come from to be honest. I work nearly exclusively with batiks (I love them
) and never do anything different with them than I do with the cotton fabrics.
My guess for these rules is that perhaps when batiks first came out they needed special handling, but the quality of batiks out now....I have never seen or ran into any issues with them. I would recommend just treating them like good quality quilting cotton unless you run into a problem.