Size of onion sets

Is there any correlation between the size of the set and the mature onion?

I saw something on youtube that suggested that the smaller firmer sets produced the best onions and that any set that had started to grow should be rejected as it will bolt. The other curious thing that was said was that it was counterproductive to put sets in too early as if the weather did turn cold and a bit frosty then bolting was likely to occur. I grow from seeds and it works for me.