I think I used English size 14 - that is about 2mm, for the last ones, for a three fold yarn (I twisted together three threads) - I have used finer - size 16 and a two thread yarn, but they did take a long time to knit.

I think 16s would be about 1.5mm, and the yarn half the thickness of double knitting, maybe less, as cotton yarn is rather dense.

Anne the Pleater