Generally, you choose a needle size based on the yarn you are using. You should see a recommended needle size and gauge on the yarn packaging. Once you pick a needle size you would make a gauge swatch to see how big your stitches are. After you figure out your gauge, you can use my Knit and Crochet Calculators to help you determine how many stitches to cast on based on the size measurement. As far as what length circular you will need, you just want to make sure that the number of stitches fit comfortably on the cord. It can be shorter than your cast on width since the stitches on the cord will be packed together. I hope this helps!