What is the correct size for an onion in recipes? Some recipes call for small, medium and large onions. Compared to what?
In general, a large onion is about the size of a 1-cup measuring cup, a medium onion would be about the size of a half-cup measure, and a small onion would be about the size of 1/4 cup.
Or a small onion could be a different beast all together, such as a shallot (the small, pointed onions with gold/purple skins) or a spring onion.
But notice the important words in that description: "In general" and "about." In most cases, onion size isn't a big deal. Onion is an ingredient that can be added based on what the cook prefers.
If a recipe depends on a more precise amount of onion, it will usually specify the amount - " 1/2 cup chopped onion" rather than just "half a large onion."
In most recipes, changing the amount or using a different type of onion is part of customizing the dish and making it your own. And that's what we call "cooking."