Grown chickens can easily eat whole corn kernels without it being cracked as long as they have access to grit. Don't worry about the size of the kernel at all.
There is usually a lot of corn in the chicken feed. It has been ground up with all the other stuff so they cannot pick and choose which grains they want but are kind of forced to have a balanced diet whe they eat it. Like any other treat, if you give them too much corn, it will upset that balanced diet they get from the feed. The general rule of thumb is to only give them enough of a treat that they can clean it up within 10 to 20 minutes. That way their diet stays well enough balanced.
I assure you, many of us violate that 10 to 20 minute rule of thumb and the chickens don't suffer. They may not be quite as efficient in laying eggs but as long as you don't go overboard with any treat, including corn, they will be OK. That's the main intent of the 10 to 20 minute rule, to keep you fronm going overboard. Often, and especially with corn, chickens will choose to eat that instead of their regular feed if it is available.
You'll read a lot about corn on this forum. In spite of what you read, corn does have a lot of nutrient value. But it is not the perfect food, so it needs to be mixed in the proper proportions with other foods to give them a balanced diet of proteins, fats, fiber, amino acids, vitamins, minerals, and whatever else they need.
