I assume it's similar to what you would do with knitting. I knit on the looser size, so I use a smaller needle. Conversely, if you knit tightly, you would go with a larger size. If you're making clothing that is fitted, you need to do a swatch--switching the needle until you get the gauge that you need. If it's a scarf, afghan, etc., you want to be close--but it's not as important. Being consistent is what matters.