What do you mean "throw out"? You throw the handfuls on the ground, scattered, and the chickens flock to it and eat it up? You might be confusing them. If this is the way you otherwise distribute treats, scratch, or other desirable foods - by tossing it on the ground for them to collect - then they might be primed to think this, too, is some kind of candy. It's like if you throw a handful of regular poultry feed on the ground, they'll think it's treats and will rush to eat it with a lot more zest than they eat it out of the feeder. Or anything fed out of a human hand can be automatically tastier than the same thing in a feeder. So don't confuse them - put the grit in a container that sits in the same place all the time. They'll learn that that's the grit container, and will go to it when they actually need grit.
As for size, I raise most of my chicks with the flock so there's already large size grit always available. When I have chicks, I just mix in chick grit together with the adult grit in the same container. All the chickens have access to it, and they can pick and choose whatever size grit they want.
