Size 5 needle is straight and 16” circular (for neck).

From Hip To Knit:

This vest is truly for everybody. Its personality can change dramatically by choice of yarn, moving its look from feminine to masculine, dressy to sporty. Choice of size can give it the look of being on the skinny-fitting fashion edge or comfortably easy fitting for grandpa. Replace the ribs with rolls and soften up the edges. Lengthen or shorten the body. A classic vest is a timeless piece of knitting.