The size of the vegetable chop doesn't really matter since they will be puréed.
Think about the strength of the vegetable's flavor; some vegetables, like zucchini, have a subtle flavor and you can add a lot, while others, like onion, have a strong flavor, so you don't want to overdo it.
If you just throw these vegetables in a pot of boiling water with a bit of salt and purée them, it will taste decent. Adding different levels of seasoning or stocks makes it better.
Add more stock or water if needed during cooking, as the liquid will reduce.
Check for chunks after blending to ensure a smooth texture.
You can add croutons, a touch of cream, or chives to your finished soup.