http://www.the-chicken-chick.com/2013/08/for-strong-eggshells-size-matters/
This is an interesting article that states studies (including this one: http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1516-635X2005000200001&lng=en&tlng=en) have found the size of the supplement is very important because shell formation tends to happen overnight, so the larger particle size will be digested/dissolved more slowly, providing the hen with a continuous source.
This is an interesting article that states studies (including this one: http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1516-635X2005000200001&lng=en&tlng=en) have found the size of the supplement is very important because shell formation tends to happen overnight, so the larger particle size will be digested/dissolved more slowly, providing the hen with a continuous source.