Note that this pattern is nearly 100 years old, so obviously the yarn and hook aren’t available anymore. A D hook is labeled 3 and an E hook is labeled 4; I couldn’t find anything labeled 3.5, & so used an E hook (which is 3.5mm). I also used sport weight yarn. With this combo, one finished “sock” (more like a bootie) was about 3 1/2” long, which is standard for newborn to 3 months size.