A product can be varied in size or quantity. A "Château Margot 1978" could, for example, be sold by the bottle, the pitcher or the glass. It works with wine but also for a fried hamburger (your burger can be of classic size or XL, XXL with a different quantity of meat).
To create a variation, go to your menu item, click on the tab "Variation" and then "Add a variation". Give a title to the variation (example: Burger size XXL), add a price, and a photo if you wish for more detail.