I know it's tough to tell because I didn't think of putting a ruler in the photo, but from left to right: chick, pullet and hen grit.
The hen grit I have varies in size and shape but is roughly the size of a small/medium corn kernel.
Unlike dogs they don't need extra vaccinations.
You'll start seeing possible roos as they get older. Once they're 8 weeks or older, you can post photos here: https://www.backyardchickens.com/forums/what-breed-or-gender-is-this.15/ of suspect birds. Look for things such as larger, redder combs...