A single-piece packaging plate foldable to form boxes having different sizes is provided for a product having a variable size. The packaging plate has an elongated plate portion, two side plate portions and two wing portions. The elongated plate portion has a plurality of parallel and spaced-apart preformed fold lines. The side plate portions extend outward from two sides of the elongated portion, and have fold lines at the junctures thereof. The two wing portions can be torn or cut out optionally. Each wing portion has a main wing extending from two sides of an area between sixth and seventh fold lines of the elongated plate portion. The packaging plate is foldable along the fold lines to form a unit body-packaging box or a system-packaging box.