Choose size based on the child’s measurements. Smilla has a close, but not tight fit: 0 cm ease in chest width, which is why you need to use a fabric with at least 30% stretch. Use the stretch guide in the pattern to evaluate your fabric.
The length is measured from the shoulder down to the hemline on a Smilla with a straight front bodice. If you sew the curved front bodice, note that it will be 2,5-3,5 cm (1 - 1 3/8”) shorter in center front.