The pattern is sized as follows:

1 - (0-3 months) 18 inch chest
2 - (3-6 months) 20 inch chest
3 - (6-9 months) 22 inch chest

Instructions are for the smallest size, with larger sizes in parentheses. When one number is given, it applies to all sizes.

From Simple Knits for Cherished Babies:
An ideal gift for a special occasion, this pretty stocking (stockinette) stitch cardigan is knitted in silk or mercerized cotton. The pearly sheen of the yarn is emphasized with a delicate rosebud trip or mother-of-pearl buttons. The front neck band is knitted integrally, giving it a very neat finish. Knit it up in bronze or gray for a small boy, without the rosebud trim. The decreasing stitches on the raglan shoulders are knitted two stitches in from the edge to create an attractive fully fashioned detail and a neater edge for sewing up.