If you're knitting in the round, the circ needs to be smaller than the circumference of the item. Unless you're knitting something really small, then you'd use 2 circs or a long one and the Magic Loop method. If you're knitting flat, it needs to be long enough to hold the sts for the item without them being crowded. So for a blanket, you wouldn't use a 16", that's too short, but for ar 36" wide one, you can use a 24-32" one.
sue- knitting heretic