choose a size that is on the smaller size to your hand measured around the palm without the thumb.
Any fingering weight yarn will work. I like a nylon in my yarn to help with the wear but you might prefer something different and thats also ok.
It is not a mistake that you use a smaller for the body and larger needle for the rib. you might even want to use a 3mm for the cast on if you struggle to get a stretchy cast on.