As you obviously know, chickens come in different sizes and eggs come in different sizes. A bantam may struggle to cover just a few regular sized eggs where a larger hen can cover a lot more. And it is not just covering the eggs that is important. The hen has to be able to cover the chicks for a while and they grow pretty fast. In the summer when it is warm covering the chciks is a lot less important than covering them in the winter.
I've seen a hen cover 18 eggs she laid herself in a hidden nest. That was decades ago and I can't remember how many she actually hatched, but it was most of them. Last year I had a hen raise 15 chicks in the summer and not lose any. In winter, I'd cut back on those numbers.
As a rule, I'd be comfortable giving a hen at least 12 eggs of the size she would normally lay herself, regardless of the weather. I would expect a hatchery Buff Orp to be able to handle a dozen Buff Orp eggs, whether those eggs come from hatchery Buff Orps or a breeder's Buff Orps. I really don't think you are going to see any significant difference in the size of those eggs unless the breeder has really been working on increasing egg size.
