Plus-size is mainly a clothes word. Plus-size clothes are cut for people whose bodies are larger than the "standard" sizes a shop keeps on the main rail — bigger measurements, more room. Shops, brands, and magazines use it as a size range, the way they say petite or tall. You will hear plus-size clothing, a plus-size model, a plus-size section. It is trying to sound matter-of-fact, even kind, rather than like an insult; though actually, not everyone likes the label, because it still marks a body as extra, as after the normal sizes. Younger people and fashion writing use it a lot in English. It is not a medical term and not a precise number: where "average" ends and "plus" begins changes by country, brand, and year.