Size of onion sets

I am not sure really. I do throw away the tiny ones though, they don't look like they'd amount to anything (which would indicate that actually, I do think there is!    )

Don't know if this is actually true BUT I had read somewhere that if you buy a bag of onion sets and open them out onto the table the ones that will actually do best are those that are not too large  The ones that will bolt are said to be the biggest ones but in general you should be trying to sort out the ones that are firm and not too big. So say in a bag where the size is 12 - 22 mm the ones nearer the 12mm that are firm will be best. Could be Bollox though 

I do not think size matters, just that they should be nice and firm. Picked up a packet in a garden centre that were virtually all sprouting so left them on the shelf. Mentioned that I was not buying them at the till only to be told that they should be planted by now. I resisted a reply.