How finely to grind egg shells?

[catesquire]

How finely do folks grind up egg shells/oyster shells to feed to your hens? Is around 1/4"-1/2" fragments good (that's what I get just taking a potato masher to them), or does it need to be smaller? Are sharp edges an issue?

Also, when do you start providing it? My ladies are just hitting four weeks, so I figured I'd start offering it soon so they can eat what they need before those first eggs come.

 

[ChickenCanoe]

I think you are good. I used to get it as fine as possible only because I wanted to avoid them eating anything that resembled part of an egg. That in an effort to avoid training egg eaters. But really, I don't think that matters.
Oyster shell and eggshells are both calcium carbonate (CaCO₃) with nearly equal percentages of calcium (Ca).
Large particles of either tend to remain in the digestive tract longer and contact the calcium absorption sites in the small intestine at night when the timing of the egg production cycle usually has the egg in the uterus where the shell is being formed.

 

[Nabiki]

I make the pieces small enough for them to eat. If you see a lot of larger pieces that don't get eaten, you know that those are too big.

I would start giving them the calcium at around 6 weeks.