Something medium-sized is not small and not large — it sits in the middle for size. A medium-sized dog is bigger than a tiny lap dog and smaller than a huge guard dog. Shops use it for clothes, boxes, cups, and portions. It is a plain, everyday adjective, not fancy and not unkind. Before a noun, careful English likes the hyphen: a medium-sized car. After the verb you can say “the car is medium-sized,” or often just “medium.” Medium by itself already does a lot of this work (“a medium coffee”), so medium-sized is the form you reach for when you want to be extra clear that you mean size. Though actually, medium has other lives in English — a newspaper, a material for painting — so the -sized ending pins this one to how big a thing is.