What sizes you buy depends entirely on the range of stones you most commonly set...
There is a proportion that looks good to the eye.
I am not fond of beads that cover a quarter of the stone, nor tiny beads precariously holding large stones. (just because you can do it - does not mean you should)
And worst of all: are those squashed mashed beads that are commonly seen done by those who overenthusiastically use power assisted tools!
Why bother to "raise" the metal to form a "bead" - and then proceed to pound it flat?