Blissy kids sizes are made to relate closely to age, a 5 year old wears size 5 etc.
The boys t-shirts are a unisex style. They are square cut and designed to sit loosely on the body.
The girls t-shirts are a fitted style and to accomodate this the cotton has a stretch.
The best way to get exact sizing is to measure a t-shirt that fits against the measurements in the table below. Remember to compare them to a t-shirt of a similar style. That is, square cut or fitted.

