Huge problem here is it doesn't matter what size you order, as most retailers seem to think of a number and stick that on as a size. If you have any of the old ones you could measure those but the number is really meaningless. Its a bit like hook sizes in that respect. I keep a few sizes in stock that I know will cover the range of bead heads I'm normally asked to tie and just re stock from the same retailer. I'm in a position to buy them by the thousand so that's a huge advantage. Both on cost and consistency.
It doesn't help that people measure beads in different ways. Some across the diameter some will measure across the edges of the hole. Just to further muddle things up they will not tell you how they have arrived at the number.
As with hooks it is really our fault for accepting this. If we agree a standard set of definitions for sizing and refuse to buy anything that isn't rated according to the standard definition manufacturers will very quickly get their act together. (That doesn't mean they have to be made to the standard, just the difference stated on the packaging so you know what you are dealing with).
Sorry I can't be of more help.