If all they eat is chick starter, they do not need grit. If they eat about anything else, they need grit. Obviously, yogurt and boiled egg yolk don't require grit, but for anything that has any texture they need it. I'm guessing the size you got is for adult birds, since younger smaller chicks can't necessarily handle the bigger chunks.
I give mine sand the second or third day in the brooder, as soon as I am sure they are eating their regular food. I usually scrape some up from my run or my gravel driveway. Construction sand works great. You want sand that has large irregular shaped grains. Avoid play sand because that is ground too fine and smooth to do any good.
I do it for different reasons. I think it sets their gizzard and digestive system up the way it should be. It is probably coincidence but pasty butt seems to go away when I start the grit. It is probably just that they are getting older and better settled in, but who knows.
If they have grit, I can give them treats if I wish. They are also prepared if my wife stuns another wasp and puts it in the brooder so they can play keep away before they eat it, in case a hard-shelled bug wanders into the brooder, or if they decide to munch on wood shavings. If they have grit, I think they are better prepared so I have less to worry about.
Another reason I use stuff out of the run is to expose them to any diseases that might be in the ground. There is some risk in this, but I obviously think more benefits since I do it. It is possible there is some disease in your ground that will wipe them out as chicks but that older birds may be able to survive. I've never had that problem, but I'm sure it could happen. There are some diseases, coccidiosis being an excellent example, that a young chick can develop an immunity to but it will more likely kill an older chicken. I think I get a healthier chicken if they are exposed to things early while I can watch them closely in a brooder and treat as necessary and they can develop their immunities.
I'm not going to say that my is the only way or necessarily the best way for you. I'll just say it is the way I do it and give some of the reasons. Lots of people do it different ways and have success. Good luck however you decide.
I started my older chicks on grit at 3 weeks but I'll start my Isa babies sooner. I use regular poultry grit that I put through my kitchen strainer to find the little bits. As far as I'm concerned, the sooner you can give them treats and kitchen scraps the better. I've had a hard time getting my 6 month old chickens to eat scraps because they weren't used to them.
I've read on a hatchery's website that you should sprinkle fine grit on top of their chick starter for the babies. If I were going to offer my adult birds grit (I don't, because they free range) I would offer it in a seperate container.
If I recall correctly, I used store bought grit one time - with my first flock - and have never bothered with it again. I get my chicks out on the ground early, around two weeks old, and they find their own grit.