Many times a recipe calls for "one onion, chopped". Onions vary in size, so what exactly does that mean? Well, it's a matter of personal taste. Evaluate your shopping habits; you probably almost always buy onions that are similar in size. Pick the size that is most common in your kitchen, chop it, and measure volume in a measuring cup. If you're satisfied with the amount of flavor that size onion gives a dish calling for one onion, that's your go-to; if you think it adds too much onion flavor, make a habit of buying smaller onions. Conversely, buy larger onions if you want more flavor.