Width is measured across the chest below the sleeves from seam to seam. Height is measured from the bottom of the shirt to the highest point on the shoulder (where the seam meets the neck).

Please note that each product has its own specific sizing chart.
SizeWidth (inches)Height (inches)
Infant 12M - Light Blue1011.5
Infant 18M - Light Blue10.512.5
Infant 24M - Light Blue10.513