It looks like you are trying to use the wrong Shirt Size Class if the size is Small. To use Small for the Shirt Size Value, you need to set the Shirt Size Class to Alpha. Numeric is used when the Shirt Size Value is a number.

The Shirt Size to Range field should be left blank if the size isn't a range. You use that field for sizing like 0-3 months with 0 months in the Shirt Size Value field and 3 months in the Shirt Size to Range field.