All measurements are of the garment itself, laid flat. Chest and waist are the flat width — double them for the full circumference. Tolerance of up to 1cm (0.4in) is standard.
Sizes are labelled by age in months and in US toddler sizes (3T, 4T) — the same conventions used in the US, so no conversion is needed.
The easiest way to pick: lay a similar garment that fits well flat at home and compare. Between sizes? We recommend sizing up.
Oversized, boxy fit — unisex design and sizing. It runs generously, so check the chest width against a tee you already own rather than going by letter size.