Story sizing and the "Uncertainty Principle"

I often have conversations with Teams about sizing, and more and more, I come back to uncertainty being perhaps the most important gauge for sizing. We use Fibonacci numbers, and when asked by people, "How do I size a story?", I go something like this... 1 - is the smallest size, and a 1 story has the characteristics of being quick to implement as there isn’t much coding involved AND we are certain about what needs changing and the impact. 2 - may need more work, but it is still well understood and we are still certain about what needs changing and the impact. 3 - we are introducing some uncertainty. We pretty much know what to do, and we know it’ll take a few days, but there is a greater risk of unknown factors impacting the work. 5 - we have a fair idea what to do, but we are likely to uncover some problems and so we are not certain about completing within a few days. 8 - there is a lot of uncertainty. It may well be best to create a “spike” story to work out what needs doing, and break in to smaller stories when we know more. Yes, we can add in other factors, and as teams become more experienced, they bring in more subtlety, but this seems a good starting point.