Best Answer: This is like asking how long is a piece of string because onions can vary in size so much. But a medium-sized onion would be something between 100 to 150 grams once it's peeled. If you're following a recipe, I would say that you don't have to get the weight of the onion exactly right, I rarely weigh onions, I just judge how much I need by eye (ie size of the onion). Source(s): Miggy · 5 years ago. Thumbs up.