I cast-on newborn size to achieve 3 months size with my gauge.
I also cast-on the stitches for both fronts and the back and knit those 3 parts together so that there was no sewing at the sides.
I did not make the stockinette part because I didn’t want to embroider the cardi.
I knitted the sleeves in the round

I stopped knitting when about to put the 5 stitches of the border on hold and bound-off on next row, 7 rows earlier than the pattern asked for.

final measurements

chest: 21 cm
height: 24 cm
sleeve: 14 cm
sleeve hem: 14 cm
neckline : 16 cm