Blazers are bought by chest size, so you will need your child's chest measurement to start. To take your child’s chest measurement, measure around the fullest part of their chest. For younger children, place the tape measure a few inches below their armpit and measure their chest there. For older children, particularly girls, the fullest part of their chest might be a little lower. Your child will be able to tell you what feels comfortable to them. Be sure not to pull the tape measure too tight as this can lead to buying shirts or blouses which are too small and strain at the buttons.