When making puréed soups, the exact chop size of vegetables doesn't matter too much, but avoid pieces so large that your blender can't chop them.
Consider the strength and subtlety of each vegetable when deciding how much to add to the soup. For example, zucchini has a mild flavor, so you can add a lot, while onion is strong, so don't overdo it.
Adding different levels of seasoning or stocks will improve the taste.
You can add croutons, a touch of cream, or chives to your soup.