A size run, sometimes called a size curve, is one of the biggest decisions in a merch order. Order too few of the popular sizes and you sell out early; order too many of the edge sizes and they sit in a box. The right curve depends on the audience, since a gym crowd, a band's fans and an office team all buy differently, and on fit, because a boxy cut or a women's style shifts demand.
Extended sizes often cost more per piece because the blanks cost more, and some styles stop at a smaller top size. Small and youth sizes may need a smaller print. Past sales, pre-order data and the size chart of the exact blank are the best guides.
Ask whether one design can be split across several sizes without extra setup, and plan reorders around your fastest-selling sizes. The size run planning guide works through examples.