This is a vintage knit pattern that was designed for medium size Boston or smooth haired fox terrier; for small schnauzer; beagle; dachshund or miniature poodle.
This is a ribbed knit dog sweater pattern knitted of sport yarn in rib stitch and fits snugly from neck to tail, with extra space at leg joints and chest. It has contrasting leg and neck bands. Knitting needles US size Nos. 1 & 5 (or English sizes 12 & 8) are used. The original photo features a Boston Terrier.
Dog's sweater size is determined by measuring your dog from the back of neck to the base of the tail. This pattern is for a medium size (chest 24", neck to tail 14", stands about 12" high).