If you’re buying food, buy the healthiest brand, like the organic one, or the one with less fat or sugar.
If you don’t know whether you will like an item, buy the smallest size.
If you know you will like it, buy the one that’s cheapest per kg or liter.
Which is usually the largest size.
If two sizes cost the same per kg, buy the smaller pack, so that you can open one and use it, while the second retains its freshness.